[Fuchsia] Don't fail for unknown architectures

[phosek]

Summary

When selecting the set of default sanitizers, don't fail for unknown
architectures. This may be the case e.g. with x86_64-unknown-fuchsia
-m32 target that's used to build the bootloader.

clang/lib/Driver/ToolChains/Fuchsia.cpp

SanitizerMask Fuchsia::getDefaultSanitizers() const {
   SanitizerMask Res;
   switch (getTriple().getArch()) {
   case llvm::Triple::aarch64:
     Res |= SanitizerKind::ShadowCallStack;
     break;
   case llvm::Triple::x86_64:
     Res |= SanitizerKind::SafeStack;
     break;
-  case llvm::Triple::riscv64:
-    break;
   default:
-    llvm_unreachable("invalid architecture");
+    // TODO: Enable SafeStack on RISC-V once tested.
+    break;
   }
   return Res;
 }