[Driver] Re-enable aarch64-cpus.c test for arm64-apple.

"darwin" is ambiguous.  When there isn't a better source
of truth (e.g., SDKs), the driver will either interpret it
as "iOS" when cross-compiling to a different architecture,
or "the host" when not.  That's now the case on AS Macs.

Update the test to more explicitly test the OS.
aarch64-mac-cpus.c already tests the mac-specific driver logic.