Darwin: Future-proof -mmacosx-version-min

Darwin: Future-proof -mmacosx-version-min

f18cbc1ee1f4 (2021-12-18) updated various parts of gcc to not impose a
Darwin or macOS version maximum of the current known release. Different
parts of gcc accept, variously, Darwin version numbers matching
darwin2*, and macOS major version numbers up to 99. The current released
version is Darwin 21 and macOS 12, with Darwin 22 and macOS 13 expected
for public release later this year. With one major OS release per year,
this strategy is expected to provide another 8 years of headroom.

However, f18cbc1ee1f4 missed config/darwin-c.c (now .cc), which
continued to impose a maximum of macOS 12 on the -mmacosx-version-min
compiler driver argument. This was last updated from 11 to 12 in
11b967577483 (2021-10-27), but kicking the can down the road one year at
a time is not a viable strategy, and is not in line with the more recent
technique from f18cbc1ee1f4.

Prior to 556ab5125912 (2020-11-06), config/darwin-c.c did not impose a
maximum that needed annual maintenance, as at that point, all macOS
releases had used a major version of 10. The stricter approach imposed
since then was valuable for a time until the particulars of the new
versioning scheme were established and understood, but now that they
are, it's prudent to restore a more permissive approach.

gcc/ChangeLog:

	* config/darwin-c.cc: Make -mmacosx-version-min more future-proof.

Signed-off-by: Mark Mentovai <mark@mentovai.com>
(cherry picked from commit 6725f186cb70d48338f69456864bf469a12ee5be)