roll xcode to 12.4

XCode 12 requires Mac OS 10.15, so update dimensions.
There's an XCode 12.5, but that requires Mac OS 11.

Silence new warning -Wpoison-system-directories,
which I've not yet found a way to work around.

Change-Id: Id49c43365e5e7e97a87fe05d82fb6f26ce1f2adc
Reviewed-on: https://skia-review.googlesource.com/c/skcms/+/374164
Reviewed-by: Eric Boren <borenet@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
3 files changed