Use soft-fp fma for MicroBlaze (bug 13304).

Systems without floating-point exceptions and rounding modes should
use the soft-fp versions of fmaf and fma, not the sysdeps/ieee754
versions that rely on setting rounding to zero and testing for the
"inexact" exception; this has been noted on
<https://sourceware.org/glibc/wiki/PortStatus> for some time.  This
patch makes MicroBlaze use the soft-fp files; sfp-machine.h is made to
include the nios2 version of sfp-machine.h which seems sufficiently
generic for 32-bit systems.

	[BZ #13304]
	* sysdeps/microblaze/s_fma.c: New file.
	* sysdeps/microblaze/s_fmaf.c: Likewise.
	* sysdeps/microblaze/sfp-machine.h: Likewise.