Xcode: Restore support for CMAKE_OSX_ARCHITECTURES=$(ARCHS_STANDARD)

Skip the `CMAKE_OSX_ARCHITECTURES` validation added by commit db409e5e68
(APPLE: Check if compilers respect CMAKE_OSX_ARCHITECTURES, 2024-05-04,
v3.30.0-rc1~15^2) if the value contains a placeholder, since we do not
know the real value.

Fixes: #26128
4 files changed