Darwin : Adjust handling of MACOSX_DEPLOYMENT_TARGET for macOS 11.
The shift to macOS version 11 also means that '11' without any
following '.x' is accepted as a valid version number. This adjusts
the validation code to accept this and map it to 11.0.0 which
matches what the clang toolchain appears to do.
gcc/ChangeLog:
* config/darwin-driver.c (validate_macosx_version_min): Allow
MACOSX_DEPLOYMENT_TARGET=11.
(darwin_default_min_version): Adjust warning spelling to avoid
an apostrophe.