cmd/5g, cmd/5l, cmd/6g, cmd/6l, cmd/8g, cmd/8l, cmd/gc, runtime: generate pointer maps by liveness analysis

This change allows the garbage collector to examine stack
slots that are determined as live and containing a pointer
value by the garbage collector.  This results in a mean
reduction of 65% in the number of stack slots scanned during
an invocation of "GOGC=1 all.bash".

Unfortunately, this does not yet allow garbage collection to
be precise for the stack slots computed as live.  Pointers
confound the determination of what definitions reach a given
instruction.  In general, this problem is not solvable without
runtime cost but some advanced cooperation from the compiler
might mitigate common cases.

R=golang-dev, rsc, cshapiro
CC=golang-dev
https://golang.org/cl/14430048