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: 630301b0d59d ("selftests/bpf: Add basic USDT selftests")
Suggested-by: Weining Lu <[e-mail]>
Suggested-by: Li Chen <[e-mail]>
Signed-off-by: Tiezhu Yang <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Reviewed-by: Huacai Chen <[e-mail]>
Cc: [e-mail]
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/[e-mail]

tools/testing/selftests/bpf/sdt.h

···
 # define STAP_SDT_ARG_CONSTRAINT        nZr
 # elif defined __arm__
 # define STAP_SDT_ARG_CONSTRAINT        g
+# elif defined __loongarch__
+# define STAP_SDT_ARG_CONSTRAINT        nmr
 # else
 # define STAP_SDT_ARG_CONSTRAINT        nor
 # endif