Reinstate gas em=freebsd for sparc-freebsd

In commit c9098af41e3 I over-simplified the sparc target decoding,
missing the fact that prior to that patch sparc-*-freebsd fell through
to the generic *-*-freebsd match.

	* configure.tgt: Add sparc-*-freebsd case.

(cherry picked from commit c24d0e8d4850d4913a1630a53e272c930849406d)