test: skip mulhilo subcase on clang<18 RVV (hangs under qemu)

clang 17's RVV backend miscompiles the new __riscv_vmulh/__riscv_vmulhu
instantiations under -mrvv-vector-bits=zvl at vlen=128: the build
succeeds but the test binary makes no forward progress under qemu-riscv64,
hitting the 6h GHA job timeout. clang >= 18 and gcc are unaffected.

Author: DiamonDinoia

test/test_batch.cpp

@@ -1198,10 +1198,16 @@ TEST_CASE_TEMPLATE("[batch]", B, BATCH_TYPES)
         Test.test_incr_decr();
     }
 
+#if !(defined(__riscv_vector) && defined(__clang__) && __clang_major__ < 18)
+    // clang < 18 miscompiles __riscv_vmulh / __riscv_vmulhu under -mrvv-vector-bits=zvl
+    // at vlen=128, causing the resulting test binary to hang under qemu-riscv64
+    // (build succeeds, runtime never makes progress). Skip the subcase on that
+    // toolchain combination; gcc and clang >= 18 cover RVV here.
     SUBCASE("mulhilo")
     {
         Test.test_mulhilo();
     }
+#endif
 
     SUBCASE("saturated_arithmetic")
     {