Fix incorrect extraction of signed constants in nios2 disassembler.

Fix incorrect extraction of signed constants in nios2 disassembler.

2018-09-23  Sandra Loosemore  <sandra@codesourcery.com>

	opcodes/
	* nios2-dis.c (nios2_print_insn_arg): Make sure signed conversions
	are used when extracting signed fields and converting them to
	potentially 64-bit types.