[gn build] Add libclang_rt.ios.a, libclang_rt.iossim.a to the build

It's built with just-built clang, like all other compiler-rt parts
in the GN build.

This requires adding some cross build support to the mac toolchain.

Also add explicit mmacosx-version-min and miphoneos-version-min
flags to the build.

ios.a is only built with the arm64 slice, iossim.a only with the
x86_64 slice for now. (The latter should maybe become host_cpu
when Arm Macs become a common iOS development platform.)

With this, it's possible to build chromium/iOS with a GN-built LLVM.

Differential Revision: https://reviews.llvm.org/D89260