cmd/bpf2go: Make LoongArch a supported target

In #975, someone added support for the loong64 GOARCH, but did't
provide a Linux string for it. Since the upstream support ([0])
has already landed, let's add the missing Linux string and make
LoongArch a supported target.

  [0]: torvalds/linux@00883922ab40

Signed-off-by: Hengqi Chen <hengqi.chen@gmail.com>

cmd/bpf2go/main.go

@@ -53,7 +53,7 @@ var targetByGoArch = map[string]target{
 	"amd64p32":    {"bpfel", ""},
 	"arm":         {"bpfel", "arm"},
 	"arm64":       {"bpfel", "arm64"},
-	"loong64":     {"bpfel", ""},
+	"loong64":     {"bpfel", "loongarch"},
 	"mipsle":      {"bpfel", ""},
 	"mips64le":    {"bpfel", ""},
 	"mips64p32le": {"bpfel", ""},