Support multilib-aware target lib flags self-specs overriding

Support multilib-aware target lib flags self-specs overriding

This patch introduces -fmultiflags, short for multilib TFLAGS, as an
option that does nothing by default, but that can be added to TFLAGS
and mapped to useful options by driver self-specs.

for  gcc/ChangeLog

	* common.opt (fmultiflags): New.
	* doc/invoke.texi: Document it.
	* gcc.cc (driver_self_specs): Discard it.
	* opts.cc (common_handle_option): Ignore it in the driver.