[PowerPC] Provide XL-compatible vec_round implementation

The XL implementation of vec_round for vector double uses
"round-to-nearest, ties to even" just as the vector float
`version does. However clang and gcc use "round-to-nearest-away"
for vector double and "round-to-nearest, ties to even"
for vector float.

The XL behaviour is implemented under the __XL_COMPAT_ALTIVEC__
macro similarly to other instances of incompatibility.

Differential revision: https://reviews.llvm.org/D113642