Include .extern and .hidden in x86-64 asm.

This is backport of BoringSSL's d216b71f909fe56255813dab0a8d052534bdcb91
and https://boringssl-review.googlesource.com/3810 and should allow asm
on x86-64 to build correctly.

Change-Id: Id321768930182951223dbf90c4c910e24d9b6798
12 files changed