Fixing float32's pround halfway criteria to match STL's criteria.

Currently packetmath_1 test is failing on Power. The reason is that pround is using vec_round which translates to vrfin, from the ISA we can see that the halfway approach is as follows

Round to Nearest
Choose the value that is closer to Z (Z1 or Z2). In case of a tie, choose the one that is even (least significant bit 0).

which is not the same as STL hence the fail.

I also changed the way to define float macros which used an incorrect approach with vec_splat_s32.