riscv, bpf: Unify 32-bit sign-extension to emit_sextw

For code unification, add emit_sextw wrapper to unify all the 32-bit
sign-extension operations.

Signed-off-by: Pu Lehui <pulehui@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Björn Töpel <bjorn@rivosinc.com>
Acked-by: Björn Töpel <bjorn@kernel.org>
Link: https://lore.kernel.org/bpf/20240115131235.2914289-2-pulehui@huaweicloud.com