runtime: fix breakpoint in ppc64x

Currently runtime.Breakpoint generates a SIGSEGV in ppc64.
The solution is an unconditional trap similar to what clang and gcc do. It is documented in the section C.6 of the ABI Book 3.

Fixes #52101

Change-Id: I071d2f2679b695ef268445b04c9222bd74e1f9af
GitHub-Last-Rev: fff4e5e
GitHub-Pull-Request: #52102
Reviewed-on: https://go-review.googlesource.com/c/go/+/397554
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Paul Murphy <murp@ibm.com>
Run-TryBot: Paul Murphy <murp@ibm.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/runtime/asm_ppc64x.s

@@ -106,7 +106,7 @@ DATA runtime·mainPC+0(SB)/8,$runtime·main<ABIInternal>(SB)
 GLOBL runtime·mainPC(SB),RODATA,$8
 
 TEXT runtime·breakpoint(SB),NOSPLIT|NOFRAME,$0-0
- MOVD R0, 0(R0) // TODO: TD
+ TW $31, R0, R0
  RET
 
 TEXT runtime·asminit(SB),NOSPLIT|NOFRAME,$0-0