Darwin: Darwin 20 is to be macOS 11 (Big Sur).

As per Nigel Tufnel's assertion "... this one goes to 11".

The various parts of the code that deal with mapping Darwin versions
to macOS (X) versions need updating to deal with  a major version of
11.

So now we have, for example:

Darwin  4 => macOS (X) 10.0
…
Darwin 14 => macOS (X) 10.10
...
Darwin 19 => macOS (X) 10.15

Darwin 20 => macOS  11.0

Because of the historical duplication of the "10" in macOSX 10.xx and
the number of tools that expect this, it is likely that system tools will
allow macos11.0 and/or macosx11.0 (despite that the latter makes little
sense).

Update the link test to cover Catalina (Darwin19/10.15) and
Big Sur (Darwin20/11.0).

gcc/ChangeLog:

	* config/darwin-c.c: Allow for Darwin20 to correspond to macOS 11.
	* config/darwin-driver.c: Likewise.

gcc/testsuite/ChangeLog:

	* gcc.dg/darwin-minversion-link.c: Allow for Darwin19 (macOS 10.15)
	and Darwin20 (macOS 11.0).