- 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>