feat(asm): take the loong64 register-pair spellings the toolchain parses

Assisted-by: GLM 5.3 Flash

verify/l64_groundtruth_test.go

@@ -28,6 +28,7 @@ func TestGroundTruthLOONG64(t *testing.T) {
 		"../testdata/verify/movwfp_loong64.s",
 		"../testdata/verify/branchu_loong64.s",
 		"../testdata/verify/atomics_loong64.s",
+		"../testdata/verify/pairs_loong64.s",
 		"../testdata/verify/vector_loong64.s",
 		"../testdata/verify/pcalign_loong64.s",
 		"../testdata/verify/l64forms_loong64.s",