[PowerPC] Implement trap and conversion builtins for XL compatibility

This patch implements trap and FP to and from double conversions. The builtins
generate code that mirror what is generated from the XL compiler. Intrinsics
are named conventionally with builtin_ppc, but are aliased to provide the same
builtin names as the XL compiler.

Differential Revision: https://reviews.llvm.org/D103668