With config.sub now properly returning m68hc12-unknown-elf rather than
m68hc12-unknown-none, more ELF tests run.  This patch enables
STB_GNU_UNIQUE processing fixing some testsuite failures that probably
no m68hc12 user cares about, and removes some XPASSes.

bfd/
	* elf32-m68hc1x.c (elf32_m68hc11_post_process_headers): Call
	_bfd_elf_post_process_headers.
ld/
	* testsuite/ld-discard/extern.d: Remove m68hc12 xfail.
	* testsuite/ld-discard/start.d: Likewise.
	* testsuite/ld-discard/static.d: Likewise.