When cross-compiling builtins, hardcode the fact that targeting MACOSX
enables JSCVT. This is useful when cross-compiling from a x86 Mac where
the compiler itself does not define __ARM_FEATURE_JSCVT for the
mksnapshot binary.

Change-Id: I2817f18de42b18f6236e5d5f26d574a0955a7557
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2637228
Commit-Queue: Pierre Langlois <pierre.langlois@arm.com>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72319}