Returns R14 and R15 to the available register pool.
Plays more nicely with ELF ABI C code.
In particular, our signal handlers will no longer crash
when a signal arrives during execution of a cgo C call.
Fixes#720.
R=ken2, r
CC=golang-dev
https://golang.org/cl/1847051
and make 5c, 8c use them.
centralizes reachability analysis
and switch generation.
now 8c doesn't have spurious warnings
in pkg/runtime.
R=ken
OCL=31266
CL=31266
