[PowerPC] Implement XL compat __fnabs and __fnabss builtins.

[PowerPC] Implement XL compat __fnabs and __fnabss builtins.

This patch implements the following floating point negative absolute value
builtins that required for compatibility with the XL compiler:
```
double __fnabs(double);
float __fnabss(float);
```

These builtins will emit :
- fnabs on PWR6 and below, or if VSX is disabled.
- xsnabsdp on PWR7 and above, if VSX is enabled.

Differential Revision: https://reviews.llvm.org/D125506