cmd/gc: support for "portable" optimization logic

Code in gc/popt.c is compiled as part of 5g, 6g, and 8g,
meaning it can use arch-specific headers but there's
just one copy of the code.

This is the same arrangement we use for the portable
code generation logic in gc/pgen.c.

Move fixjmp and noreturn there to get the ball rolling.

R=ken2
CC=golang-dev
https://codereview.appspot.com/12789043