darwin, d: Support outfile substitution for libphobos

In the gdc driver, this takes the previous fix for the Darwin D
bootstrap, and extends it to the -static-libphobos option as well.
Rather than pushing the -static-libphobos option back onto the command
line, the setting of SKIPOPT is instead conditionally removed.  The same
change has been repeated for -static-libstdc++ so there is now no need
to call generate_option to re-add it.

In the gcc driver, -static-libphobos has been added as a common option,
validated, and a new outfile substition added to config/darwin.h to
correctly replace -lgphobos with libgphobos.a.

gcc/ChangeLog:

	* common.opt (static-libphobos): Add option.
	* config/darwin.h (LINK_SPEC): Substitute -lgphobos with libgphobos.a
	when linking statically.
	* gcc.c (driver_handle_option): Set -static-libphobos as always valid.

gcc/d/ChangeLog:

	* d-spec.cc (lang_specific_driver): Set SKIPOPT on -static-libstdc++
	and -static-libphobos only when target supports LD_STATIC_DYNAMIC.
	Remove generate_option to re-add -static-libstdc++.

libphobos/ChangeLog:

	* testsuite/testsuite_flags.in: Add libphobos library directory as
	search path to --gdcldflags.