RISC-V: Fix the error when building RISC-V linux native gdbserver.

RISC-V: Fix the error when building RISC-V linux native gdbserver.

The original report is as follow,
https://sourceware.org/pipermail/binutils/2020-June/111383.html

Inlcude the bfd.h in the include/opcode/riscv.h may cause gdbserver fail
to build.  I just want to use the `bfd_boolean` in the opcodes/riscv-opc.c,
but I didn't realize this cause the build failed.  Fortunately, I can also
use the `int` as the function return types just like others in the
opcodes/riscv-opc.c.

	include/
	* opcode/riscv.h: Remove #include "bfd.h".  And change the return
	types of riscv_get_isa_spec_class and riscv_get_priv_spec_class
	from bfd_boolean to int.

	opcodes/
	* riscv-opc.c (riscv_get_isa_spec_class): Change bfd_boolean to int.
	(riscv_get_priv_spec_class): Likewise.