libgrust: 'AM_ENABLE_MULTILIB' only for target builds [PR113056]

..., but not for host builds, which don't need it, and it may cause the build
to fail.

Use what appears to be the standard pattern for that (see
'libbacktrace/configure.ac', 'zlib/configure.ac').

	PR rust/113056
	libgrust/
	* configure.ac: 'AM_ENABLE_MULTILIB' only for target builds.
	* configure: Regenerate.