Suffered from a confusion of masm/marmasm syntax for the x64 host and
arm64 target (could only generate one syntax or ther other). Fixed by
moving the compile-time flag to a runtime one.

Bug: v8:10012

Change-Id: I34746a495b1881c1d0465995930979bb768b07e6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1962854Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Richard Townsend <richard.townsend@arm.com>
Cr-Commit-Position: refs/heads/master@{#66650}