Improve COFF/PE linker garbage collection by preventing the removal of sections conta...

Improve COFF/PE linker garbage collection by preventing the removal of sections containing exported symbols.

	PR ld/19803
	* ldlang.c (lang_add_gc_name): New function.  Adds the provided
	symbol name to the list of gc symbols.
	(lang_process): Call lang_add_gc_name with entry_symbol_default if
	entry_symbol.name is NULL.  Use lang_add_gc_name to add the init
	and fini function names.
	* pe-dll.c (process_def_file_and_drectve): Add exported names to
	the gc symbol list.
	* testsuite/ld-pe/pr19803.s: Do not export _testval symbol.
	* testsuite/ld-pe/pr19803.d: Tweak expected output.