[GCC] Match a GCC version with a patch suffix without a third version component

Previously it would only accept a string as a GCC version if it had
either two components and no suffix, or three components with an
optional suffix.

Debian and ubuntu provided mingw compilers have lib/gcc/target entries
like "5.3-posix" and "5.3-win32". This doesn't try to make any specific
preference between them (other than lexical sorting of the suffix).

Differential Revision: https://reviews.llvm.org/D45505

llvm-svn: 330696