opcodes/riscv-dis.c: Make XLEN variable static

opcodes/riscv-dis.c: Make XLEN variable static

Before changing the core disassembler, we take care of minor code clarity
issues and improve readability.

Since xlen variable is not (and should not) used outside riscv-dis.c,
this commit makes this variable static.

opcodes/ChangeLog:

	* riscv-dis.c (xlen): Make this variable static.