clang assumes 16-byte stack alignment, but incoming stack alignment isn't
always guaranteed to be that way.  It looks like v8 was lucky to not hit
this so far.

See https://crbug.com/418554 -- this makes v8's standalone config match
Chromium.  See also https://llvm.org/bugs/show_bug.cgi?id=21414

Maybe it's possible to change the caller of OnEntryHook() to guarantee
the right alignment, but matching Chromium's build flags here seems like
a good idea in general.

BUG=v8:4928
LOG=n

Review URL: https://codereview.chromium.org/1899783002

Cr-Commit-Position: refs/heads/master@{#35597}