write/elf: fix writing of strtab when symtab is empty

[philipc]

Previously, if the symtab was empty then we were writing a strtab that had 0 bytes of data. This gives a linker error:
SHT_STRTAB string table section [index 4] is empty

Also, if there is a symtab then there must be a strtab, otherwise the error is:
invalid sh_type for string table section [index 0]: expected SHT_STRTAB, but got SHT_NULL

Closes #709

[klensy]

= note: rust-lld: error: /home/user/rust/build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/release/deps/libpanic_unwind-0a0cf8f8a42e5845.rlib(lib.rmeta): invalid sh_type for string table section [index 0]: expected SHT_STRTAB, but got SHT_NULL
          collect2: error: ld returned 1 exit status

[philipc]

@klensy I've force pushed a new attempt to fix this.

[klensy]

This works (but it will be nice to actually find why it failed before, is it ld requirement?).

[philipc]

An empty strtab is always invalid, so that part is a bug in this crate, and ld rightly complained about it.

In theory it should be able to work without a strtab if there are no strings, but I haven't checked what the ELF spec says for that and my first attempt showed ld didn't support that either. A missing dynstr is valid, which is what #646 was fixing, but it incorrectly changed the strtab handling too.

The current fix in this PR is still a bit of a hack. Now that I know what's needed, I'll do a better fix and some testing tomorrow.