Fix gdb.linespec/cpls-ops.exp on 32-bit

Fix gdb.linespec/cpls-ops.exp on 32-bit

gdb.linespec/cpls-ops.exp is currently failing on x86-64 -m32 and other
32-bit ports:

 b test_op_new::operator new(unsigned int) FAIL: gdb.linespec/cpls-ops.exp: operator-new: tab complete "b test_op_new::operator" (timeout)
 ^CQuit
 (gdb) complete b test_op_new::operator
 b test_op_new::operator new(unsigned int)
 (gdb) FAIL: gdb.linespec/cpls-ops.exp: operator-new: cmd complete "b test_op_new::operator"

The problem is simply that the testcase incorrectly assumes that
size_t is "unsigned long".

Fix this by extracting the right type with the "ptype" command.

gdb/testsuite/ChangeLog:
2017-11-30  Pedro Alves  <palves@redhat.com>

	* gdb.linespec/cpls-ops.exp
	(check_explicit_skips_function_argument): Extract the underlying
	type of size_t instead of hardcoding it.