Tidy the Makefiles for devel/freebsd-gcc*. No functional change.

• Sort BUILD_DEPENDS before LIB_DEPENDS and RUN_DEPENDS.
• Quiet other warnings noted by portlint.
• Sort USES.

Requested by: mat (1), lwhsu (3)
Reviewed by: lwhsu
Differential Revision: https://reviews.freebsd.org/D29072