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
add "extern register" support to 8c.
extern register means allocate in the FS-relative segment.
make 8l generate segmented stack checks.
R=ken
OCL=26600
CL=26606