Adapt "cross compile?" check for Apple Silicon

This piece of code tries to implement the semantics "cross compile?" to
determine CFLAGS used for test binary compilation.
```
if(ANDROID OR ${arch} MATCHES "arm|aarch64|riscv32|riscv64")
```

Since Apple Silicon, macOS runs on arm64e, so we take the wrong branch
when compiling and running tests locally "on the host" on an AS machine.

Furthermore, for Apple code, we use the separate
`get_test_cflags_for_apple_platform` function to determine these test
compiliation flags and `get_test_cc_for_arch` is only ever used in the
"compile & run on host" case, so we can short-curcuit the "cross
compile?" check here.

rdar://91446703

Differential Revision: https://reviews.llvm.org/D123633