X86: convert object streamer selection to a switch

Change the object streamer selection to a switch from a series of if conditions.
Rather than defaulting to ELF, require that an ELF format is requested.  The
Windows/!ELF is maintained as MachO would have been selected first and will
still provide a MachO format.  Add an assertion that if COFF is requested that
the target platform is Windows as only WinCOFF object emission is currently
supported.

llvm-svn: 207200