_xgetbv is reserved for the implementation and shouldn't be used by user
code. Newer GCCs and clang trunk define _xgetbv, leading to a name
collision if xsaveintrin.h gets included transitively.

This unbreaks building v8 with clang trunk and libstdc++ 4.9, which
happens to pull in xsaveintrin.h transitively through <algorithm>. Newer
versions of libstdc++ don't seem to do that anymore which is why this
issue never showed up before.

R=bmeurer@chromium.org

Change-Id: If94efaf4798e5420738064bcbf26880f904c76a9
Reviewed-on: https://chromium-review.googlesource.com/c/1414858Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58849}