cpu: replace use of the reserved _xgetbv identifier with xgetbv.

Clang built from trunk now implements _xgetbv, causing errors as it is
now redefined.

Modeled after https://crrev.com/c/1414858 which is a similar fix to v8.

TEST=built locally with clang from trunk

Change-Id: Ie1c242ae6b9ae34b695a512991e90a74dfafdedc
Reviewed-on: https://chromium-review.googlesource.com/c/1431352
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Jordan Rupprecht <rupprecht@google.com>
Cr-Commit-Position: refs/heads/master@{#625379}
1 file changed