Follow morestack, so that crashes during a stack split
give complete traces.  Also mark stack segment boundaries
as an aid to debugging.

Correct various line number bugs with yet another attempt
at interpreting the pc/ln table.  This one has a chance at
being correct, because I based it on reading src/cmd/ld/lib.c
instead of on reading the documentation.

Fixes #1138.
Fixes #1430.
Fixes #1461.

throw: runtime: split stack overflow

runtime.throw+0x3e /home/rsc/g/go2/src/pkg/runtime/runtime.c:78
        runtime.throw(0x81880af, 0xf75c8b18)
runtime.newstack+0xad /home/rsc/g/go2/src/pkg/runtime/proc.c:728
        runtime.newstack()
runtime.morestack+0x4f /home/rsc/g/go2/src/pkg/runtime/386/asm.s:184
        runtime.morestack()
----- morestack called from stack: -----
runtime.new+0x1a /home/rsc/g/go2/src/pkg/runtime/malloc.c:288
        runtime.new(0x1, 0x0, 0x0)
gongo.makeBoard+0x33 /tmp/Gongo/gongo_robot_test.go:344
        gongo.makeBoard(0x809d238, 0x1, 0xf76092c8, 0x1)
----- stack segment boundary -----
gongo.checkEasyScore+0xcc /tmp/Gongo/gongo_robot_test.go:287
        gongo.checkEasyScore(0xf764b710, 0x0, 0x809d238, 0x1)
gongo.TestEasyScore+0x8c /tmp/Gongo/gongo_robot_test.go:255
        gongo.TestEasyScore(0xf764b710, 0x818a990)
testing.tRunner+0x2f /home/rsc/g/go2/src/pkg/testing/testing.go:132
        testing.tRunner(0xf764b710, 0xf763b5dc, 0x0)
runtime.goexit /home/rsc/g/go2/src/pkg/runtime/proc.c:149
        runtime.goexit()

R=ken2, r
CC=golang-dev
https://golang.org/cl/4000053