runtime: set $sp before $pc in gdb python script

When setting $pc, gdb does a backtrace using the current value of $sp,
and it may complain if $sp does not match that $pc (although the
assignment went through successfully).

This happens with ARM SSA backend: when setting $pc it prints
> Cannot access memory at address 0x0

As well as occasionally on MIPS64:
> warning: GDB can't find the start of the function at 0xc82003fe07.
> ...

Setting $sp before setting $pc makes it happy.

Change-Id: Idd96dbef3e9b698829da553c6d71d5b4c6d492db
Reviewed-on: https://go-review.googlesource.com/23940Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/runtime-gdb.py

@@ -448,15 +448,15 @@ class GoroutineCmd(gdb.Command):
 		except gdb.error:
 			pc = int(str(pc).split(None, 1)[0], 16)
 		save_frame = gdb.selected_frame()
-		gdb.parse_and_eval('$save_pc = $pc')
 		gdb.parse_and_eval('$save_sp = $sp')
-		gdb.parse_and_eval('$pc = {0}'.format(str(pc)))
+		gdb.parse_and_eval('$save_pc = $pc')
 		gdb.parse_and_eval('$sp = {0}'.format(str(sp)))
+		gdb.parse_and_eval('$pc = {0}'.format(str(pc)))
 		try:
 			gdb.execute(cmd)
 		finally:
-			gdb.parse_and_eval('$pc = $save_pc')
 			gdb.parse_and_eval('$sp = $save_sp')
+			gdb.parse_and_eval('$pc = $save_pc')
 			save_frame.select()