[CMake] Don't modify `FUZZER_SUPPORTED_ARCH` is place.

On a Darwin host we were modifying the `FUZZER_SUPPORTED_ARCH` in place
which would strip out non-x86 architectures. This unhelpful if we
want to use `FUZZER_SUPPORTED_ARCH` later.

To fix this we introduce `FUZZER_TEST_ARCH` which is similar to what we
have for for the other sanitizers. For non-Darwin host platforms
`FUZZER_TEST_ARCH` is the same as `FUZZER_SUPPORTED_ARCH` but for Darwin
host platforms we use `darwin_filter_host_archs(...)` as the previous
code did.

llvm-svn: 359394