Move uses_elf_em to ld-lib.exp

and add a missing entry from uses_genelf.

binutils/
	* testsuite/lib/binutils-common.exp (uses_elf_em): Delete.
ld/
	* testsuite/lib/ld-lib.exp (uses_genelf): Add moxie-*-moxiebox.
	(uses_elf_em): New.