This is a reland of 3ad101f5

Original change's description:
> [assembler] Split out CPUFeatures into its own file
>
> This reduces the preprocessor expanded source size by 84,675 LoC:
>
> gen         (   20 files):    71,349 to    1,523,934 (   21x)
> src         (  624 files):   367,410 to   53,253,894 (  145x)
> test        (  392 files):   490,503 to   37,436,176 (   76x)
> third_party (  432 files):   239,085 to    9,547,902 (   40x)
> total       ( 1520 files): 1,183,031 to  102,736,424 (   87x)
>
> to
>
> gen         (   20 files):    71,349 to    1,523,794 (   21x)
> src         (  624 files):   367,411 to   53,186,896 (  145x)
> test        (  392 files):   490,504 to   37,418,639 (   76x)
> third_party (  432 files):   239,085 to    9,547,902 (   40x)
> total       ( 1520 files): 1,183,033 to  102,651,749 (   87x)
>
>
> Change-Id: Ia8a79092051a42815b65e86a0784297915368c9b
> Reviewed-on: https://chromium-review.googlesource.com/c/1291471
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
> Reviewed-by: Marja Hölttä <marja@chromium.org>
> Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#58266}

TBR=marja@chromium.org,clemensh@chromium.org,ulan@chromium.org

Change-Id: I5b857666508b1c80dcadd0b470aada37dd49077e
Reviewed-on: https://chromium-review.googlesource.com/c/1379872Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58278}