RISC-V: Fix T-Head immediate types on printing

This commit fixes two minor typing-related issues for
T-Head immediate operands.

1.  A signed type must be specified when printing with %i.
2.  unsigned/signed int is not portable enough for max 32-bit immediates.
    Instead, we should use unsigned/signed long.
    The format string is changed accordingly.

opcodes/ChangeLog:

	* riscv-dis.c (print_insn_args): Fix T-Head immediate types on
	printing.