Darwin: Truncate kernel-provided version to OS major for Darwin >= 20.

Darwin: Truncate kernel-provided version to OS major for Darwin >= 20.

In common with system tools, GCC uses a version obtained from the kernel as
the prevailing macOS target, when that is not overridden by command line or
environment versions (i.e. mmacosx-version-min=, MACOSX_DEPLOYMENT_TARGET).

Presently, GCC assumes that if the OS version is >= 20, the value used should
include both major and minium version identifiers.  However the system tools
(for those versions) truncate the value to the major version - this leads to
link errors when combining objects built with clang and GCC for example:

ld: warning: object file (null.o) was built for newer macOS version (12.2)
than being linked (12.0)

The change here truncates the values GCC uses to the major version.

gcc/ChangeLog:

	PR target/104871
	* config/darwin-driver.cc (darwin_find_version_from_kernel): If the OS
	version is darwin20 (macOS 11) or greater, truncate the version to the
	major number.

(cherry picked from commit add1adaa17a294ea25918ffb4fdd40f115362632)