Rework the Bazel workaround's relationship with linkstatic

Due to a longstanding Bazel flaw
(bazelbuild/bazel#22041), we need to split all
our mixed C/C++ targets in two. Ideally this split would behave as if
the Bazel flaw were fixed, with the split-out library statically linked
with the other source files. Accordingly, the helper macro sets
linkstatic = True.

It turns out linkstatic = True does not work this way. Bazel interprets
linkstatic such that, if A(test, linkshared) -> B(library) -> C(library,
linkstatic), C will be statically linked into A, not B. This is probably
to avoid diamond dependency problems but means it is not possible for a
cc_library split to be transparent, only cc_binary and cc_test.

So what is happening is that every cc_test that links libcrypto is
getting mlkem.cc statically linked into it, separate from the rest of
libcrypto! That means we're getting the worst of both worlds: worse
cache hit rate for tests that link libcrypto AND our C/C++ bits are
still not contained in the same library.

linkstatic = True on the helper is still valuable in cc_test and
cc_binary, but otherwise inherit the outer value.

Change-Id: I1089c58c6ddaa90c89efd8cdcebd88169b0236c8
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/71508
Reviewed-by: Bob Beck <bbe@google.com>
Commit-Queue: David Benjamin <davidben@google.com>

util/util.bzl

@@ -83,13 +83,24 @@ boringssl_copts_c = boringssl_copts_common + select({
     "//conditions:default": ["-std=c11"] + gcc_copts_c,
 })
 
+def linkstatic_kwargs(linkstatic):
+    # Although Bazel's documentation says linkstatic defaults to True or False
+    # for the various target types, this is not true. The defaults differ by
+    # platform non-Windows and True on Windows. There is now way to request the
+    # default except to omit the parameter, so we must use kwargs.
+    kwargs = {}
+    if linkstatic != None:
+        kwargs["linkstatic"] = linkstatic
+    return kwargs
+
 def handle_mixed_c_cxx(
         name,
         copts,
         deps,
         internal_hdrs,
         includes,
         linkopts,
+        linkstatic,
         srcs,
         testonly,
         alwayslink):
@@ -116,13 +127,11 @@ def handle_mixed_c_cxx(
             srcs = srcs_cxx + internal_hdrs,
             copts = copts + boringssl_copts_cxx,
             includes = includes,
-            # This target only exists to be linked into the main library, so
-            # always link it statically.
-            linkstatic = True,
             linkopts = linkopts,
             deps = deps,
             testonly = testonly,
             alwayslink = alwayslink,
+            **linkstatic_kwargs(linkstatic),
         )
 
         # Build the remainder as a C-only target.
@@ -168,16 +177,6 @@ def handle_asm_srcs(asm_srcs):
         "//conditions:default": asm_srcs,
     })
 
-def linkstatic_kwargs(linkstatic):
-    # Although Bazel's documentation says linkstatic defaults to True or False
-    # for the various target types, this is not true. The defaults differ by
-    # platform non-Windows and True on Windows. There is now way to request the
-    # default except to omit the parameter, so we must use kwargs.
-    kwargs = {}
-    if linkstatic != None:
-        kwargs["linkstatic"] = linkstatic
-    return kwargs
-
 def bssl_cc_library(
         name,
         asm_srcs = [],
@@ -199,6 +198,14 @@ def bssl_cc_library(
         internal_hdrs = hdrs + internal_hdrs,
         includes = includes,
         linkopts = linkopts,
+        # Ideally we would set linkstatic = True to statically link the helper
+        # library into main cc_library. But Bazel interprets linkstatic such
+        # that, if A(test, linkshared) -> B(library) -> C(library, linkstatic),
+        # C will be statically linked into A, not B. This is probably to avoid
+        # diamond dependency problems but means linkstatic does not help us make
+        # this function transparent. Instead, just pass along the linkstatic
+        # nature of the main library.
+        linkstatic = linkstatic,
         srcs = srcs,
         testonly = testonly,
         alwayslink = alwayslink,
@@ -250,6 +257,10 @@ def bssl_cc_binary(
         deps = deps,
         internal_hdrs = [],
         includes = includes,
+        # If it weren't for https://github.com/bazelbuild/bazel/issues/22041,
+        # the split library be part of `srcs` and linked statically. Set
+        # linkstatic to match.
+        linkstatic = True,
         linkopts = linkopts,
         srcs = srcs,
         testonly = testonly,
@@ -288,6 +299,10 @@ def bssl_cc_test(
         deps = deps,
         internal_hdrs = [],
         includes = includes,
+        # If it weren't for https://github.com/bazelbuild/bazel/issues/22041,
+        # the split library be part of `srcs` and linked statically. Set
+        # linkstatic to match.
+        linkstatic = True,
         linkopts = linkopts,
         srcs = srcs,
         testonly = True,