cmd/5g, cmd/5l, cmd/6l, cmd/8l, cmd/gc, cmd/ld, runtime: accurate args and locals information

Previously, the func structure contained an inaccurate value for
the args member and a 0 value for the locals member.

This change populates the func structure with args and locals
values computed by the compiler.  The number of args was
already available in the ATEXT instruction.  The number of
locals is now passed through in the new ALOCALS instruction.

This change also switches the unit of args and locals to be
bytes, just like the frame member, instead of 32-bit words.

R=golang-dev, bradfitz, cshapiro, dave, rsc
CC=golang-dev
https://codereview.appspot.com/7399045