Since we pre-generate our perlasm, having the output of these files be
sensitive to the environment the run in is unhelpful. It would be bad to
suddenly change what features we do or don't compile in whenever workstations'
toolchains change or if developers do or don't have CC variables set.

Previously, all compiler-version-gated features were turned on in
https://boringssl-review.googlesource.com/6260, but this broke the build. I
also wasn't thorough enough in gathering performance numbers. So, flip them all
to off instead. I'll enable them one-by-one as they're tested.

This should result in no change to generated assembly.

Change-Id: Ib4259b3f97adc4939cb0557c5580e8def120d5bc
Reviewed-on: https://boringssl-review.googlesource.com/6383
Reviewed-by: Adam Langley <agl@google.com>