[PowerPC] Fix rounding mode for vec_round in altivec.h

The function is supposed to be the equivalent of rint() (as in
round to nearest, ties to even) rather than round() (round to
nearest, ties away from zero). In fact, the instruction we emit
without VSX is vrfin which is correct. However, with VSX we emit
xvrspi which is the equivalent of round() and therefore incorrect.
Since there is no equivalent VSX instruction, simply use vrfin
regardless of availability of VSX.