* regparm.h: New file to define platform dependent regparm attributes.
	Define all of them empty if __CYGMAGIC__ is defined.
	* cygerrno.h: Include regparm.h instead of defining _reg[123] here.
	* winsup.h: Ditto.
	* cygmagic: Define __CYGMAGIC__ to help generate same result on all
	platforms.
	* child_info.h (CURR_CHILD_INFO_MAGIC): Update.
	(class child_info_fork): Drop extraneous semicolon.