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>