CL 93658 moved stack trace printing inside a systemstack call to
sidestep complexity in case the runtime is in a inconsistent state.

Unfortunately, debuggers generating backtraces for a Go panic
will be confused and come up with a technical correct but useless
stack. This CL moves just the crash performing - typically a SIGABRT
signal - outside the systemstack call to improve backtraces.

Unfortunately, the crash function now needs to be marked nosplit and
that triggers the no split stackoverflow check. To work around that,
split fatalpanic in two: fatalthrow for runtime.throw and fatalpanic for
runtime.gopanic. Only Go panics really needs crashes on the right stack
and there is enough stack for gopanic.

Example program:

package main

import "runtime/debug"

func main() {
	debug.SetTraceback("crash")
	crash()
}

func crash() {
	panic("panic!")
}

Before:
(lldb) bt
* thread #1, name = 'simple', stop reason = signal SIGABRT
  * frame #0: 0x000000000044ffe4 simple`runtime.raise at <autogenerated>:1
    frame #1: 0x0000000000438cfb simple`runtime.dieFromSignal(sig=<unavailable>) at signal_unix.go:424
    frame #2: 0x0000000000438ec9 simple`runtime.crash at signal_unix.go:525
    frame #3: 0x00000000004268f5 simple`runtime.dopanic_m(gp=<unavailable>, pc=<unavailable>, sp=<unavailable>) at panic.go:758
    frame #4: 0x000000000044bead simple`runtime.fatalpanic.func1 at panic.go:657
    frame #5: 0x000000000044d066 simple`runtime.systemstack at <autogenerated>:1
    frame #6: 0x000000000042a980 simple at proc.go:1094
    frame #7: 0x0000000000438ec9 simple`runtime.crash at signal_unix.go:525
    frame #8: 0x00000000004268f5 simple`runtime.dopanic_m(gp=<unavailable>, pc=<unavailable>, sp=<unavailable>) at panic.go:758
    frame #9: 0x000000000044bead simple`runtime.fatalpanic.func1 at panic.go:657
    frame #10: 0x000000000044d066 simple`runtime.systemstack at <autogenerated>:1
    frame #11: 0x000000000042a980 simple at proc.go:1094
    frame #12: 0x00000000004268f5 simple`runtime.dopanic_m(gp=<unavailable>, pc=<unavailable>, sp=<unavailable>) at panic.go:758
    frame #13: 0x000000000044bead simple`runtime.fatalpanic.func1 at panic.go:657
    frame #14: 0x000000000044d066 simple`runtime.systemstack at <autogenerated>:1
    frame #15: 0x000000000042a980 simple at proc.go:1094
    frame #16: 0x000000000044bead simple`runtime.fatalpanic.func1 at panic.go:657
    frame #17: 0x000000000044d066 simple`runtime.systemstack at <autogenerated>:1

After:
(lldb) bt
* thread #7, stop reason = signal SIGABRT
  * frame #0: 0x0000000000450024 simple`runtime.raise at <autogenerated>:1
    frame #1: 0x0000000000438d1b simple`runtime.dieFromSignal(sig=<unavailable>) at signal_unix.go:424
    frame #2: 0x0000000000438ee9 simple`runtime.crash at signal_unix.go:525
    frame #3: 0x00000000004264e3 simple`runtime.fatalpanic(msgs=<unavailable>) at panic.go:664
    frame #4: 0x0000000000425f1b simple`runtime.gopanic(e=<unavailable>) at panic.go:537
    frame #5: 0x0000000000470c62 simple`main.crash at simple.go:11
    frame #6: 0x0000000000470c00 simple`main.main at simple.go:6
    frame #7: 0x0000000000427be7 simple`runtime.main at proc.go:198
    frame #8: 0x000000000044ef91 simple`runtime.goexit at <autogenerated>:1

Updates #22716

Change-Id: Ib5fa35c13662c1dac2f1eac8b59c4a5824b98d92
Reviewed-on: https://go-review.googlesource.com/110065
Run-TryBot: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>