Darwin: Future-proof and homogeneize detection of darwin versions

The current GCC branch will become 12.1.0, which will be the stable
version of GCC when the next macOS version is released. There are some
places in GCC that don’t handle darwin22 as a version, so we need to
future-proof it (gcc/config.gcc and gcc/config/darwin-driver.c). We
align that code with what Apple clang does, i.e. accept all potential
major macOS versions until 99.

This patch also homogenises the handling of darwin version numbers,
where the majority of places use darwin2*, but some used darwin2[0-9]*.
Since there never was a darwin2.x version, the two are equivalent, and
we prefer the simpler darwin2*

gcc/ChangeLog:

	* config/darwin-driver.c: Make version code more future-proof.
	* config.gcc: Homogeneize darwin versions.
	* configure.ac: Homogeneize darwin versions.
	* configure: Regenerate.

gcc/testsuite/ChangeLog:

	* gcc.dg/darwin-minversion-link.c: Test darwin21.
	* obj-c++.dg/cxx-ivars-3.mm: Homogeneize darwin versions.
	* obj-c++.dg/objc-gc-3.mm: Homogeneize darwin versions.
	* objc.dg/objc-gc-4.m: Homogeneize darwin versions.