Fix extraction of signed constants in nios2 disassembler (again).

Fix extraction of signed constants in nios2 disassembler (again).

In commit 6031ac352c05c5c9f44e24fa1c5a8222a7a7d02d I added some casts
to explicitly do conversions from unsigned to signed as 32-bit
quantities to address some bugs with different sizes of long and
bfd_signed_vma.  Those casts were removed in the rewrite of the
sign-extension logic in commit 1d61b032265e69317f42e8019e072506f11890c5,
reintroducing the same bugs.  This patch restores the casts.

2020-05-26  Sandra Loosemore  <sandra@codesourcery.com>

	opcodes/
	* nios2-dis.c (nios2_print_insn_arg): Add explicit casts to
	extractions of signed fields.