LoongArch, bpf: Fix jit to skip speculation barrier opcode

Just skip the opcode(BPF_ST | BPF_NOSPEC) in the BPF JIT instead of
failing to JIT the entire program, given LoongArch currently has no
couterpart of a speculation barrier instruction. To verify the issue,
use the ltp testcase as shown below.

Also, Wang says:

  I can confirm there's currently no speculation barrier equivalent
  on LonogArch. (Loongson says there are builtin mitigations for
  Spectre-V1 and V2 on their chips, and AFAIK efforts to port the
  exploits to mips/LoongArch have all failed a few years ago.)

Without this patch:

  $ ./bpf_prog02
  [...]
  bpf_common.c:123: TBROK: Failed verification: ??? (524)
  [...]
  Summary:
  passed   0
  failed   0
  broken   1
  skipped  0
  warnings 0

With this patch:

  $ ./bpf_prog02
  [...]
  Summary:
  passed   0
  failed   0
  broken   0
  skipped  0
  warnings 0

Fixes: 5dc61552 ("LoongArch: Add BPF JIT support")
Signed-off-by: George Guo <guodongtai@kylinos.cn>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: WANG Xuerui <git@xen0n.name>
Cc: Tiezhu Yang <yangtiezhu@loongson.cn>
Link: https://lore.kernel.org/bpf/20230328071335.2664966-1-guodongtai@kylinos.cn