Add two new options for querying the compiler.

Add two new options for querying the compiler.  The first new option,
--output-target-arch, prints the target architecture.  This is useful in
situations where you are working directly with the contents of the Mercury
subdirectory and --use-grade-subdirs is enabled.  (The directory names
generated by --use-grade-subdirs include the target architecture.) The second
new option, --output-class-dir, deals with a more specific instance of the
above.  It prints out the name of the directory in which the Mercury compiler
places generated Java class files.

Replace the use of 'fullarch' by 'target_arch' throughout the compiler.
(We still currently support '--fullarch' as a synonym for '--target-arch'
but that too will eventually be dropped.)

compiler/options.m:
	Add the new options.

compiler/mercury_compile.m:
	Implement the new options.

	s/fullarch/target_arch/

	Add an XXX comment about an inaccurate error message.

compiler/handle_options.m:
compiler/compile_target_code.m:
compiler/file_names.m:
compiler/make.program_target.m:
	Conform to the above changes.

doc/user_guide.texi:
	Document the new options.