Always include x86_64-gcc.c in the standalone build.

Unlike the standalone build, builds generated from util/generate_build_files.py
do not exclude x86_64-gcc.c. Match the consumer builds by making the standalone
build unconditionally include it. (This would have noticed the missing
preprocessor checks in the file.)

Change-Id: I8d20f269dea63776320ae636ee1e5339cb85fa30
Reviewed-on: https://boringssl-review.googlesource.com/4761
Reviewed-by: Adam Langley <agl@google.com>