cmd/6g, liblink, runtime: support saving base pointers

This adds a "framepointer" GOEXPERIMENT that that makes the amd64
toolchain maintain base pointer chains in the same way that gcc
-fno-omit-frame-pointer does.  Go doesn't use these saved base
pointers, but this does enable external tools like Linux perf and
VTune to unwind Go stacks when collecting system-wide profiles.

This requires support in the compilers to not clobber BP, support in
liblink for generating the BP-saving function prologue and unwinding
epilogue, and support in the runtime to save BPs across preemption, to
skip saved BPs during stack unwinding and, and to adjust saved BPs
during stack moving.

As with other GOEXPERIMENTs, everything from the toolchain to the
runtime must be compiled with this experiment enabled.  To do this,
run make.bash (or all.bash) with GOEXPERIMENT=framepointer.

Change-Id: I4024853beefb9539949e5ca381adfdd9cfada544
Reviewed-on: https://go-review.googlesource.com/2992
Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/runtime2.go

@@ -125,6 +125,7 @@ type gobuf struct {
 	ctxt unsafe.Pointer // this has to be a pointer so that gc scans it
 	ret  uintreg
 	lr   uintptr
+	bp   uintptr // for GOEXPERIMENT=framepointer
 }
 
 // Known to compiler.