travis: Compile a more compatible libc.a for musl

The mitigations for rust-lang#34978 involve passing `-Wa,-mrelax-relocations=no` to all C
code we compile, and we just forgot to pass it when compiling musl itself.

Closes rust-lang#39979

src/ci/docker/linux-tested-targets/build-musl.sh

@@ -11,7 +11,10 @@
 
 set -ex
 
-export CFLAGS="-fPIC"
+# We need to mitigate rust-lang/rust#34978 when compiling musl itself as well
+export CFLAGS="-fPIC -Wa,-mrelax-relocations=no"
+export CXXFLAGS="-Wa,-mrelax-relocations=no"
+
 MUSL=musl-1.1.14
 curl https://www.musl-libc.org/releases/$MUSL.tar.gz | tar xzf -
 cd $MUSL