Apparently Windows' .lib files are also ar. Add tests.

Change-Id: Ie35f410268086b8fe6d4d1b491de3f30a46309dd
Reviewed-on: https://boringssl-review.googlesource.com/c/33348
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>

One less bit of special-casing in read_symbols.go. We filter out the
sysv-style symbol table, so we should filter out the macOS one too.

Add tests for util/ar to cover this and the Linux case.

Change-Id: Id16d8b0526c1b6e0149df1df4006848d7b3a4b2f
Reviewed-on: https://boringssl-review.googlesource.com/c/33347
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>

- Add util/read_symbols.go to read exported symbols from an AR
  file for use with the symbol prefixing feature
- Move util/fipstools/fipscommon/ar.go -> util/ar/ar.go
- util/ar/ar.go: Support BSD-style AR files

Change-Id: I171b3b952e69c4b87ac04751b7dba3ea9bc2504e
Reviewed-on: https://boringssl-review.googlesource.com/32024
Reviewed-by: David Benjamin <davidben@google.com>