[PowerPC] Add support for embedded devices with EFPU2

PowerPC cores like e200z759n3 [1] using an efpu2 only support single precision
hardware floating point instructions. The single precision instructions efs*
and evfs* are identical to the spe float instructions while efd* and evfd*
instructions trigger a not implemented exception.

This patch introduces a new command line option -mefpu2 which leads to
single-hardware / double-software code generation.

[1] Core reference:
  https://www.nxp.com/files-static/32bit/doc/ref_manual/e200z759CRM.pdf

Differential revision: https://reviews.llvm.org/D92935