MIPS: math-emu: Add support for the MIPS R6 SELNEZ FPU instruction

MIPS R6 introduced the following instruction:
SELNEZ.fmt: FPR[fd]  FPR[ft].bit0 ? FPR[fs] : 0

Add support for emulating the single and double precision
formats of the said instruction.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/10955/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>