Fix --link-object <archive> with --restricted-command-line.

When --restricted-command-line is enabled, we try to shorten the link
command by creating a temporary archive of object files, including files
specified with --link-object.  --link-object can also be used to specify
archives (.a) to link with, which results in nested archives in the
temporary archive.  The linker does not look for object files in nested
archives so it does not work as intended.

compiler/compile_target_code.m:
	Create the temporary archive for --restricted-command-line
	consisting of only the file names in ObjectsList with known
	object file extensions.  Pass any other names in ObjectsList
	to the link command directly.
Document that --link-object and MLOBJS may specify archives.

It was not explicitly stated that --link-object and MLOBJS may specify
archives to link with.  The Mercury build system uses that fact, the
documentation contains some examples doing so, and it has been a
consideration since at least commit eadcd7ab00ca5599f4bc371803afd5b106c581f6,
so document it as a supported feature.

compiler/options.m:
doc/reference_manual.texi:
doc/user_guide.texi:
	Mention archives in --link-object and MLOBJS documentation.