gdb/riscv: Add equality operators to riscv_gdb_features

gdb/riscv: Add equality operators to riscv_gdb_features

Add '==' and '!=' operators for the struct riscv_gdb_features,
allowing a small simplification.

gdb/ChangeLog:

	* arch/riscv.h (riscv_gdb_features::operator==): New.
	(riscv_gdb_features::operator!=): New.
	* riscv-tdep.c (riscv_gdbarch_init): Make use of the inequality
	operator.