RISC-V: Print XTheadMemPair literal as "immediate"

The operand type "Xl(...)" denotes that (...) is a literal.  Specifically,
they are intended to be a constant immediate value.

This commit prints "Xl(...)" operand with dis_style_immediate style,
not dis_style_text.

opcodes/ChangeLog:

	* riscv-dis.c (print_insn_args): Use dis_style_immediate on
	the constant literal of the "Xl..." operand.