[lto][patch] add -flto-single option

[Rafael Espindola]

I have update Bill's patch to the current branch. All that was
necessary was fixing a trivial merge conflict. The patch looks to be
working as intended. It is very handy to test programs outside of the
gcc testsuite before we are able to easily combine multiple files.

OK for LTO if it bootstraps and tests are OK?

	* gcc.c: (lto_single): New variable.  Flag to indicate
	lto-single mode.
	(LINK_COMMAND_SPEC,cc1_options): Add support for
	lto-single mode.
	(invoke_lto_single): New constant.
	Specs string to run hidden lto1 pass.
	(default_compilers): Add support for lto-single
	mode in specs for C compilations.
	(static_specs): Add entry for invoke_lto_single.
	(process_command): Recognize -flto-single option.
	(handle_spec_predicate): Parse and invoke spec predicate,
	similar to spec funtion.
	(handle_braces): Implement conditional on spec predicates.
	(lto_single_spec_predicate): New function. Spec predicate
	to test for lto-single mode.

Cheers,
-- 
Rafael Avila de Espindola

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