devel/freebsd-gcc{9,12}: Various cleanups, no functional change

• Use DISTVERSION instead of PORTVERISON
• Don't silence post-install commands to remove various files from the staging area
• Simplify logic to handle arch-specific packing lists
• Clarify package comment and description
• Clarify FreeBSD 12 ignore rule for gcc12

Reviewed by: fuz
Differential Revision: https://reviews.freebsd.org/D42590