config.gcc: Remove default for xm_file.

config.gcc: Remove default for xm_file.

	* config.gcc: Remove default for xm_file.  Clean up local
	machine type handling.  Remove settings of xm_file to
	something that doesn't exist.  In the big switch, set xm_file,
	don't append to it (the value before the big switch is always
	null).  Don't force build_xm_file or host_xm_file to have a value.
	* configure.in: Remove default for xm_file.  Do not special
	case $cpu/xm-$cpu.h not existing.
	* configure: Regenerate.

From-SVN: r41106