Forcefully disable RVV support for now.

Index: deps/v8/third_party/highway/src/hwy/detect_compiler_arch.h
--- deps/v8/third_party/highway/src/hwy/detect_compiler_arch.h.orig
+++ deps/v8/third_party/highway/src/hwy/detect_compiler_arch.h
@@ -266,7 +266,7 @@
 #define HWY_ARCH_WASM 0
 #endif
 
-#ifdef __riscv
+#if 0
 #define HWY_ARCH_RISCV 1
 #else
 #define HWY_ARCH_RISCV 0
