ld: Add --no-print-map-discarded option

ld: Add --no-print-map-discarded option

Add a new option to disable the listing of discarded sections
in map file output.  The use case stems from a large application
built with -ffunction-sections --gc-sections where the list of
discarded sections blows up the map file output.  The default
behaviour remains to print discarded sections,  but the new option
allows us to disable it.

ld/
	* NEWS: Mention new option --no-print-map-discarded.
	* ld.h (ld_config_type) <print_map_discarded>: New field.
	* ldlang.c (lang_map): Conditionally output discarded sections
	in map files based on configuration option.
	* ldlex.h (option_values) <OPTION_PRINT_MAP_DISCARDED,
	OPTION_NO_PRINT_MAP_DISCARDED>: New.
	* ldmain.c (main): Enabled print_map_discarded by default.
	* lexsup.c (ld_options): Add new command-line options.
	(parse_args) <OPTION_NO_PRINT_MAP_DISCARDED,
	OPTION_PRINT_MAP_DISCARDED>: New cases.
	* ld.texi: Document new options.
	* testsuite/ld-gc/gc.exp: Add new test.
	* testsuite/ld-gc/skip-map-discarded.s: New file.
	* testsuite/ld-gc/skip-map-discarded.d: New file.
	* testsuite/ld-gc/skip-map-discarded.map: New file.