Detecting AVX2 requires the use of a sub-leaf.
To make things easier we remove our use of the cpuid.h
implementation on GCC and just use our existing inline asm
implementation.
The bulk of this commit was generated by running:
run-clang-tidy.py \
-checks='-*,llvm-namespace-comment' \
-header-filter=^/.../mozilla-central/.* \
-fix