Improve broken-on-gcc checks

Mark powerpc* and riscv broken.

Refactor and add a TRY_GCC_BROKEN option to build anyway. This
simplifies things for people trying to get gcc builds working
while letting other developers know that they aren't expected to work.

Reviewed by: jhb, emaste
Improves: 4c0dfd5959bd4 arm: fail early on gcc builds
Differential Revision: https://reviews.freebsd.org/D45230