selftests/bpf: Use asm constraint "m" for LoongArch

Currently, LoongArch LLVM does not support the constraint "o" and no plan
to support it, it only supports the similar constraint "m", so change the
constraints from "nor" in the "else" case to arch-specific "nmr" to avoid
the build error such as "unexpected asm memory constraint" for LoongArch.

Fixes: 630301b0 ("selftests/bpf: Add basic USDT selftests")
Suggested-by: Weining Lu <luweining@loongson.cn>
Suggested-by: Li Chen <chenli@loongson.cn>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Huacai Chen <chenhuacai@loongson.cn>
Cc: stable@vger.kernel.org
Link: https://llvm.org/docs/LangRef.html#supported-constraint-code-list
Link: https://github.com/llvm/llvm-project/blob/main/llvm/lib/Target/LoongArch/LoongArchISelDAGToDAG.cpp#L172
Link: https://lore.kernel.org/bpf/20241219111506.20643-1-yangtiezhu@loongson.cn