preparePanic must set all registers expected by Go runtime conventions
in case the sigpanic is being injected into C code. However, on
mips64x it fails to restore RSB (R28). As a result, if C code modifies
RSB and then raises a signal that turns into a sigpanic call, sigpanic
may crash when it attempts to lock runtime.debuglock (the first global
it references).

Fix this by restoring RSB in the signal context using the same
convention as main and sigtramp.

Fixes #23641.

Change-Id: Ib47e83df89e2a3eece10f480e4e91ce9e4424388
Reviewed-on: https://go-review.googlesource.com/91156
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>