This adds support for iOS builds in libsampler. Both iOS simulator
builds (target architecture x64) and iOS device builds (arm64) are
supported.

Note that this is mostly untested since we neither have iOS bots nor
an iOS test runner. This CL was thus only tested by compiling V8 for
both iOS simulator & device targets.

Bug: v8:9140
Change-Id: Ib618bf793771f4be84d1979a968d2b3ef9f6ff86
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1569436Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60898}