Fix ICE due to condition mismatch between expander and define_insn.

ice.i:7:1: error: unrecognizable insn:
    7 | }
      | ^
(insn 7 6 8 2 (set (reg:V2SF 84 [ vect__3.8 ])
        (unspec:V2SF [
                (reg:V2SF 86 [ vect__1.7 ])
                (const_int 11 [0xb])
            ] UNSPEC_ROUND)) "ice.i":5:14 -1
     (nil))
during RTL pass: vregs

gcc/ChangeLog:

	PR target/107970
	* config/i386/mmx.md (btruncv2sf2): Add TARGET_MMX_WITH_SSE to
	the condition.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/pr107970.c: New test.