Fix scm-ports.exp regression

Fix scm-ports.exp regression

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.

What happens is that on x86, this:

	set sp_reg [get_integer_valueof "\$sp" 0]

... ends up setting sp_reg to a negative value, because
get_integer_valueof uses "print/d":

    print /d $sp
    $1 = -11496

Then later the test suite does:

    gdb_test "guile (print (seek rw-mem-port (value->integer sp-reg) SEEK_SET))" \
	"= $sp_reg" \
	"seek to \$sp"

... expecting this value to be identical to the saved $sp_reg value.
However it gets:

    guile (print (seek rw-mem-port (value->integer sp-reg) SEEK_SET))
    = 4294955800

"print" is just a wrapper for guile's format:

    gdb_test_no_output "guile (define (print x) (format #t \"= ~A\" x) (newline))"

The seek function returns a scm_t_off, the printing of which is
handled by guile, not by gdb.

Tested on x86-64 Fedora 26 using an ordinary build and also a -m32
build.

2018-01-15  Tom Tromey  <tom@tromey.com>

	* gdb.guile/scm-ports.exp (test_mem_port_rw): Use get_valueof to
	compute sp_reg.