Fix ld/x86: reduce testsuite dependency on system object files

commit eab996435fe65a421541f59557c5f1fd427573a3
Author: Jan Beulich <jbeulich@suse.com>
Date:   Tue Nov 7 13:58:32 2023 +0100

    ld/x86: reduce testsuite dependency on system object files

changed some C compiler tests to assembler/linker tests which introduced
2 problems:

1. It broke x32 binutils tests since --64 was passed to assembler, but
-m elf_x86_64 wasn't passed to linker.
2. -nostdlib was passed to C compiler driver to exclude standard run-time
files which should be avoided with -r option for linker tests.

Fix them by passing -m elf_x86_64 to linker and removing -nostdlib for
linker tests with -r.

	PR ld/30722
	* testsuite/ld-x86-64/x86-64.exp: Pass -m elf_x86_64 to linker
	for tests with --64.  Remove -nostdlib for tests with -r.