cmd/link/internal/ld: fix DWARF offsets with GOEXPERIMENT=framepointer

The offsets computed by the DWARF expressions for local variables
currently don't account for the extra stack slot used by the frame
pointer when GOEXPERIMENT=framepointer is enabled.

Fix this by adding the extra stack slot to the offset.

This fixes TestGdbPython with GOEXPERIMENT=framepointer.

Updates #15840.

Change-Id: I1b2ebb2750cd22266f4a89ec8d9e8bfa05fabd19
Reviewed-on: https://go-review.googlesource.com/23458
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/link/internal/ld/dwarf.go

@@ -1558,6 +1558,12 @@ func writelines(prev *LSym) *LSym {
 				if !haslinkregister() {
 					offs -= int64(SysArch.PtrSize)
 				}
+				if obj.Framepointer_enabled != 0 {
+					// The frame pointer is saved
+					// between the CFA and the
+					// autos.
+					offs -= int64(SysArch.PtrSize)
+				}
 
 			case obj.A_PARAM:
 				dt = DW_ABRV_PARAM