cmd/cc, cmd/gc, runtime: emit bitmaps for scanning locals.

Previously, all word aligned locations in the local variables
area were scanned as conservative roots.  With this change, a
bitmap is generated describing the locations of pointer values
in local variables.

With this change the argument bitmap information has been
changed to only store information about arguments.  The locals
member, has been removed.  In its place, the bitmap data for
local variables is now used to store the size of locals.  If
the size is negative, the magnitude indicates the size of the
local variables area.

R=rsc
CC=golang-dev
https://codereview.appspot.com/12328044