ld/xc16x: This target does not support -shared

ld/xc16x: This target does not support -shared

Add the xc16x-elf target to the list of targets that do not support
the -shared option.  Being missing from this list was causing the
linker to add '-z norelro' to the link line of many tests, which in
turn caused these tests to fail.

ld/ChangeLog:

	* testsuite/lib/ld-lib.exp (check_shared_lib_support): Add
	xc16x-*-elf to the list of targets that don't support -shared.