Update to the 20171130 snapshot of GCC 7.2.1.

Backport the change to ensure what we install is stripped (i.e., debug
info is removed) from lang/gcc7. See revision 454177 for background. [1]

Shorten the distinct .if statements for architecture-specific settings,
to use .elif instead.

Reported by: Ports QA Framework, miwi, sobomax [1]
Discussed with: tijl, miwi [1]
Differential Revision: https://reviews.freebsd.org/D10357 [1]