[SPARC64]: Fix quad-float multiply emulation.

Something is wrong with the 3-multiply (vs. 4-multiply) optimized
version of _FP_MUL_MEAT_2_*(), so just use the slower version
which actually computes correct values.

Noticed by Rene Rebe

Signed-off-by: David S. Miller <davem@davemloft.net>