Support: use internal `call_once` on PPC64le

Use the replacement execute once threading support in LLVM for PPC64le.  It
seems that GCC does not define `__ppc__` and so we would actually call out to
the C++ runtime there which is not what the current code intended.  Check both
`__ppc__` and `__PPC__`.  This avoids the need for checking the endianness.

Thanks to nemanjai for the hint about GCC's behaviour and the fact that the
reviewed condition could be simplified.

Original patch by Sarvesh Tamba!

llvm-svn: 348970