LoongArch: BPF: Implement PROBE_MEM32 pseudo instructions

LoongArch: BPF: Implement PROBE_MEM32 pseudo instructions

Add support for `{LDX,STX,ST} | PROBE_MEM32 | {B,H,W,DW}` instructions.
They are similar to PROBE_MEM instructions with the following differences:
 * PROBE_MEM32 supports store.
 * PROBE_MEM32 relies on the verifier to clear upper 32-bit of the
  src/dst register
 * PROBE_MEM32 adds 64-bit kern_vm_start address (which is stored in S6
  in the prologue). Due to bpf_arena constructions such S6 + reg +
  off16 access is guaranteed to be within arena virtual range, so no
  address check at run-time.
 * S6 is a free callee-saved register, so it is used to store arena_vm_start
 * PROBE_MEM32 allows ST and STX. If they fault the store is a nop. When
  LDX faults the destination register is zeroed.

To support these on LoongArch, we employ the t2/t3 registers to store the
intermediate results of reg_arena + src/dst reg and use the t2/t3 registers
as the new src/dst reg. This allows us to reuse most of the existing code.

Acked-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Tested-by: Vincent Li <vincent.mc.li@gmail.com>
Signed-off-by: Hengqi Chen <hengqi.chen@gmail.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>