testsuite: Fix up new51.C test on various targets [PR108533]

testsuite: Fix up new51.C test on various targets [PR108533]

The test fails on targets where size_t is not unsigned long
due to extra diagnostics.

As the testcase is tested in C++98 too, I'm not using decltype (sizeof 0)
but __SIZE_TYPE__.

2023-01-25  Jakub Jelinek  <jakub@redhat.com>

	PR c++/107329
	PR testsuite/108533
	* g++.dg/init/new51.C (size_t): New typedef.
	(RexxClass::operator new, RexxClass::operator delete): Use size_t
	instead of unsigned long.