MIPS: IPL is 8bit in Cause and Status registers if TARGET_MCU

MIPS: IPL is 8bit in Cause and Status registers if TARGET_MCU

If MIPS MCU extension is enable, the IPL section in Cause and Status
registers has been expand to 8bit instead of 6bit.

In Cause: the bits are 10-17.
In Status: the bits are 10-16 and 18.

MD00834-2B-MUCON-AFP-01.03.pdf: P49 and P61.

gcc/ChangeLog:

	* config/mips/mips.cc (mips_expand_prologue):
	  IPL is 8bit for MCU ASE.