runtime: fix GDB tests on s390x running Ubuntu 18.04

On Ubuntu 18.04 I am seeing GDB fail to restore the stack pointer
during this test because stack unwinding can't find the PC. This CL
is essentially a partial revert of CL 23940 and fixes the issue on
s390x.

Change-Id: Ib4c41162dc85dc882eb6e248330f4082c3fa94c3
Reviewed-on: https://go-review.googlesource.com/c/go/+/169857
Run-TryBot: Cherry Zhang <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Cherry Zhang <[email protected]>

Author: mundaym

src/runtime/runtime-gdb.py

@@ -540,8 +540,8 @@ def invoke(self, arg, _from_tty):
 			# In GDB, assignments to sp must be done from the
 			# top-most frame, so select frame 0 first.
 			gdb.execute('select-frame 0')
-			gdb.parse_and_eval('$sp = $save_sp')
 			gdb.parse_and_eval('$pc = $save_pc')
+			gdb.parse_and_eval('$sp = $save_sp')
 			save_frame.select()