universe: Permit requesting a specific version of GCC

If USE_GCC_TOOLCHAINS is set to a value matching the pattern 'gcc*',
use that as the GCC version. For example, USE_GCC_TOOLCHAINS=gcc16
would use amd64-gcc16 for amd64, etc. If the variable is set to a
value that doesn't match that pattern, use the default version.

Reviewed by: imp, emaste
Differential Revision: https://reviews.freebsd.org/D48418