[PowerPC] Emit IR module flag for current float abi

This is part of the efforts adding .gnu_attribute support for PowerPC.
In Clang, an extra metadata field will be added as float-abi to show
current long double format. So backend can emit .gnu_attribute section
data from this metadata.

To avoid breaking existing behavior, the module metadata will only be
emitted when this module makes use of long double.

Reviewed By: nemanjai

Differential Revision: https://reviews.llvm.org/D116016