[patch, nios2] fix constant fixup overflow assembler bug

[Sandra Loosemore]

Altera reported a Nios II assembler bug to us in which a movia 
pseudo-instruction with a negative constant operand generated bad code. 
  E.g.

	movia r2, 0xfffffff0

turned into

	movhi	r3,0             <- wrong register r3 instead of r2
	addi	r2,r2,-16

Turns out this really didn't have anything to do with movia per se; the 
problem was the failure to mask constant BFD_RELOC_NIOS2_HIADJ16 fixups 
to 16 bits before or'ing them with the instruction word.  Such masking 
is indicated by the documentation for the %hiadj macro in the processor 
documentation.

I've checked in the attached patch with the obvious fix and an 
additional test case.  Since this was a fairly generic change that could 
affect things other than movia, I also regression-tested on 
gcc/g++/libstdc++ testsuites to give broader test coverage.

-Sandra

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