RISC-V: Make riscv_isa_xlen a global function.

RISC-V: Make riscv_isa_xlen a global function.

This allows the function to be used from riscv OS files, which also need to
depend on XLEN size.

	gdb/
	* riscv-tdep.c (riscv_isa_xlen): Drop static.
	* riscv-tdep.h (riscv_isa_xlen): Add extern declaration.