Fix garbage collection of common symbols

Fix garbage collection of common symbols

Running lang_common before garbage collection means slightly less work
in garbage collection code, since common symbols should no longer
appear there.  It does have the side effect of keeping linker script
symbols (at least those defined outside of sections) global too,
hence some testsuite churn.

bfd/
	PR 17165
	* elf-bfd.h (ELF_COMMON_DEF): Note that this might be true for
	linker script assignments too.
	* elflink.c (elf_gc_sweep_symbol): Don't drop ELF_COMMON_DEF syms.
	(bfd_elf_gc_mark_dynamic_ref_symbol): Similarly.
ld/
	PR 17165
	* ldlang.c (lang_process): Run lang_common before lang_gc_sections.
ld/testsuite/
	* ld-gc/pr14265.d,
	* ld-cris/tls-gc-68.d,
	* ld-cris/tls-gc-69.d,
	* ld-cris/tls-gc-70.d,
	* ld-cris/tls-gc-71.d,
	* ld-cris/tls-gc-75.d,
	* ld-cris/tls-gc-76.d,
	* ld-cris/tls-gc-79.d,
	* ld-mmix/bpo-10.d,
	* ld-mmix/bpo-11.d: Update.