LoongArch: BPF: Support small struct arguments for trampoline

LoongArch: BPF: Support small struct arguments for trampoline

In the current BPF code, the struct argument size is at most 16 bytes,
enforced by the verifier. According to the Procedure Call Standard for
LoongArch, the struct argument size below 16 bytes are provided as part
of the 8 argument registers, that is to say, the struct argument may be
passed in a pair of registers if its size is more than 8 bytes and no
more than 16 bytes.

Extend the BPF trampoline JIT to support attachment to functions that
take small structures (up to 16 bytes) as argument, save and restore a
number of "argument registers" rather than a number of arguments.

With this patch, the following related testcases passed:

  sudo ./test_progs -a tracing_struct/struct_args
  sudo ./test_progs -a tracing_struct/union_args

Link: https://github.com/loongson/la-abi-specs/blob/release/lapcs.adoc#structures
Acked-by: Hengqi Chen <hengqi.chen@gmail.com>
Tested-by: Hengqi Chen <hengqi.chen@gmail.com>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>