PCScan: Use SIMD for x86_64 in the scanning loop

This CL tries to use avx2 and sse3 for scanning and performing
IsInGigaCage() check.

Performance (Intel(R) Xeon(R) Gold 6154):
pa:pcscan:browser_process:scan   sse3: -16%  avx2: -25%.
pa:pcscan:browser_process        sse3: -14%  avx2: -21%.

Bug: 11297512
Change-Id: I669d835cded23393fa2fc8a7f10170b86568ad8d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2644804


Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#847435}