Disable jiterp safepoints in single-threaded WASM (#99558)

Safepoints don't do anything in single threaded wasm, so the jiterp can just not generate any code for them. This should address part of a performance regression in #99554

src/mono/browser/runtime/jiterpreter-support.ts

@@ -1246,12 +1246,14 @@ class Cfg {
             this.overheadBytes += 11;
         }
 
-        // Account for the size of the safepoint
-        if (
-            (branchType === CfgBranchType.SafepointConditional) ||
-            (branchType === CfgBranchType.SafepointUnconditional)
-        ) {
-            this.overheadBytes += 17;
+        if (WasmEnableThreads) {
+            // Account for the size of the safepoint
+            if (
+                (branchType === CfgBranchType.SafepointConditional) ||
+                (branchType === CfgBranchType.SafepointUnconditional)
+            ) {
+                this.overheadBytes += 17;
+            }
         }
     }
 
@@ -1505,6 +1507,9 @@ export const _now = (globalThis.performance && globalThis.performance.now)
 let scratchBuffer: NativePointer = <any>0;
 
 export function append_safepoint(builder: WasmBuilder, ip: MintOpcodePtr) {
+    // safepoints are never triggered in a single-threaded build
+    if (!WasmEnableThreads)
+        return;
     // Check whether a safepoint is required
     builder.ptr_const(cwraps.mono_jiterp_get_polling_required_address());
     builder.appendU8(WasmOpcode.i32_load);