runtime: improvements for signal registers
runtime: improvements for signal registers

Fix dumpregs on i386, implement dumpregs for PPC Linux/AIX, get PC on AIX.

Change-Id: Icaf4ba0be72575f71818ae916abefe08d2d2739c
Reviewed-on: https://go-review.googlesource.com/56170
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/gofrontend,golang/gofrontend,golang/gofrontend,golang/gofrontend