[PowerPC] Emit gnu_attribute according to float-abi metadata

According to GNU as documentation, PowerPC supports some .gnu_attribute
tags to represent the vector and float ABI type in the object file.
Some linkers like GNU ld respects the attribute and will prevent objects
with conflicting ABIs being linked.

This patch emits gnu_attribute value in assembly printer according to
the float-abi metadata. More attributes for soft-fp, hard single/double
and even vector ABI need to be supported in the future.

Reviewed By: jsji

Differential Revision: https://reviews.llvm.org/D117193