[RFA] Fix scm-ports.exp regression

[Doug Evans via gdb-patches]

Tom Tromey writes:
 > In https://sourceware.org/ml/gdb-patches/2017-12/msg00215.html, Jan
 > pointed out that the scalar printing patches caused a regression in
 > scm-ports.exp on x86.
 > 
 > I think the simplest fix is to use "print/u" rather than "print/d" to
 > get the value of sp_reg in the test case.
 > 
 > Tested on x86-64 Fedora 26 using an ordinary build and also a -m32
 > build.
 > 
 > 2018-01-03  Tom Tromey  <tom@tromey.com>
 > 
 > 	* gdb.guile/scm-ports.exp (test_mem_port_rw): Use get_valueof to
 > 	compute sp_reg.

LGTM