aarch64, Darwin: Set a default cpu type when none is configured.

We just choose apple-m1 here, since that should work on all released
arm64 [macOS] systems (so far). We are still not handling iOS/iPhoneOS
so this might need revision should we eventually support them.

gcc/ChangeLog:

	* config.gcc (aarch64-*-darwin*): Default to apple-m1 CPU.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>

gcc/config.gcc

@@ -1237,6 +1237,8 @@ aarch64-*-darwin* )
 	tmake_file="${tmake_file} aarch64/t-aarch64 aarch64/t-aarch64-darwin"
 	tm_defines="${tm_defines} TARGET_DEFAULT_ASYNC_UNWIND_TABLES=1"
 	tm_defines="${tm_defines} DISABLE_AARCH64_AS_CRC_BUGFIX=1"
+	# Choose a default CPU version that will work for all current releases.
+	with_cpu=${with_cpu:-apple-m1}
 	;;
 aarch64*-*-freebsd*)
 	tm_file="${tm_file} elfos.h ${fbsd_tm_file}"