x86-64: Test protected function pointers

x86-64: Test protected function pointers

On x86-64, function pointers in executable for external funtions may be
resolved to their PLT entries in executable.  If it happens, function
pointers of protected funtions in shared libraries must be resolved to
the PLT entries in executable, not addresses of protected funtions in
shared libraries.

	PR ld/27973
	* testsuite/ld-x86-64/x86-64.exp: Run protected function tests.
	* testsuite/ld-x86-64/protected-func-1.h: New file.
	* testsuite/ld-x86-64/protected-func-1a.s: Likewise.
	* testsuite/ld-x86-64/protected-func-1b.c: Likewise.