Wrap a few opcodes headers in extern "C" for C++

Wrap a few opcodes headers in extern "C" for C++

These are sufficient to link an --enable-targets=all GDB build in C++
mode, on x86_64 Fedora 20.

include/opcode/
2015-02-19  Pedro Alves  <palves@redhat.com>

	* cgen.h [__cplusplus]: Wrap in extern "C".
	* msp430-decode.h [__cplusplus]: Likewise.
	* nios2.h [__cplusplus]: Likewise.
	* rl78.h [__cplusplus]: Likewise.
	* rx.h [__cplusplus]: Likewise.
	* tilegx.h [__cplusplus]: Likewise.

opcodes/
2015-02-19  Pedro Alves  <palves@redhat.com>

	* microblaze-dis.h [__cplusplus]: Wrap in extern "C".