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

Author: seehearfeel

tools/testing/selftests/bpf/sdt.h

@@ -102,6 +102,8 @@
 # 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