Replace CallNativeSafe round-trips with fcvtl/fcvtl2 for half-to-float
conversion. A rev64 fixup corrects the PPC word-pair ordering. FLOAT16_2
uses ext to rotate the two source halfs into position and sets the
constant {0.0, 1.0} tail lanes. PACK_FLOAT16 kept as C helpers since
fcvtn doesn't match Xenos overflow saturation semantics.
8911a3b7d8