Darwin : Update the kernel version to macOS version mapping.

Darwin : Update the kernel version to macOS version mapping.

With the change to macOS 11 and Darwin20, the algorithm for mapping
kernel version to macOS version has changed.

We now have darwin 20.X.Y => macOS 11.(X > 0 ? X - 1 : 0).??.
It currently unclear if the Y will be mapped to macOS patch version
and, if so, whether it will be one-based or 0-based.
Likewise, it's unknown if Darwin 21 will map to macOS 12, so these
entries are unchanged for the present.

gcc/ChangeLog:

	* config/darwin-driver.c (darwin_find_version_from_kernel):
	Compute the minor OS version from the minor kernel version.