gcc.c: Move data on prefixes forward in file and reorganize.

gcc.c: Move data on prefixes forward in file and reorganize.

	* gcc.c: Move data on prefixes forward in file and reorganize.
	(md_exec_prefix, md_startfile_prefix, md_startfile_prefix_1):
	Always define, but make null if no value.
	(static_specs): Add "md_exec_prefix", "md_startfile_prefix",
	and "md_startfile_prefix_1".
	(main): Check whether md_exec_prefix and the others are the
	null string rather than whether the macro is defined.

From-SVN: r36730