s12z and h8300 no-print-map-discarded fails

s12z and h8300 no-print-map-discarded fails

This tidies the remaining --no-print-map-discarded fails.  h8300-elf
warns on a section without flags, and s12z doesn't support
--gc-sections.

bfd/
	* elf32-s12z.c (elf_backend_can_gc_sections): Don't define
ld/
	* testsuite/ld-gc/skip-map-discarded.s: Add section attributes.
	* testsuite/lib/ld-lib.exp (check_gc_sections_available): Add
	s12z to list of targets not supporting --gc-sections.