This will allow to remove some duplicated code (i.e. thread synchronization)
while at same time removing unnecessary use of inline ASM for Intel features
detection.

A few other advantages:
 - remove some extra logic (e.g. no need to test the platform to include the
   correct CPU detection header).
 - simplifies the buildsystem (i.e. we always include cpu_features.c)
 - get rid of the simd_stub file.

Bug: 1032721
Change-Id: I9427b34ec09dddc41925844a6ec4e6aa4d8f3207
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1987190


Reviewed-by: Mike Klein <mtklein@chromium.org>
Reviewed-by: Adenilson Cavalcanti <cavalcantii@chromium.org>
Commit-Queue: Adenilson Cavalcanti <cavalcantii@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#729515}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: d989ac2c596e88e7581eb93d053945a43c611124