Darwin, config: Amend for Darwin 21 / macOS 12.

It seems that the OS major version is now tracking the kernel
major version - 9.  Minor version has been set to kerne
min - 1.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>
Signed-off-by: Saagar Jha <saagar@saagarjha.com>

gcc/ChangeLog:

	* config.gcc: Adjust for Darwin21.
	* config/darwin-c.c (macosx_version_as_macro): Likewise.
	* config/darwin-driver.c (validate_macosx_version_min):
	Likewise.
	(darwin_find_version_from_kernel): Likewise.