RISC-V: move some stray __RISCV_INSN_FUNCS definitions from kprobes

RISC-V: move some stray __RISCV_INSN_FUNCS definitions from kprobes

The __RISCV_INSN_FUNCS originally declared riscv_insn_is_* functions inside
the kprobes implementation. This got moved into a central header in
commit ec5f90877516 ("RISC-V: Move riscv_insn_is_* macros into a common header").

Though it looks like I overlooked two of them, so fix that. FENCE itself is
an instruction defined directly by its own opcode, while the created
riscv_isn_is_system function covers all instructions defined under the SYSTEM
opcode.

Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Heiko Stuebner <heiko.stuebner@vrull.eu>
Link: https://lore.kernel.org/r/20230113211955.3534431-1-heiko@sntech.de
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>