LoongArch: BPF: Sign extend kfunc call arguments

In the Linux kernel, the following vulnerability has been resolved:

LoongArch: BPF: Sign extend kfunc call arguments

The kfunc calls are native calls so they should follow LoongArch calling conventions. Sign extend its arguments properly to avoid kernel panic. This is done by adding a new emit_abi_ext() helper. The emit_abi_ext() helper performs extension in place meaning a value already store in the target register (Note: this is different from the existing sign_extend() helper and thus we can't reuse it).