[wasm] Enable System.Numerics vectors SIMD (#78068)

This helps in cases where there's only S.N.Vector path in the BCL code, like `SpanHelpers.IndexOfChar(ref char searchSpace, char value, int length)`. In this case the `Span:IndexOf char` measurement improves from 0.0143ms to 0.0114ms.

* [wasm] Enable System.Numerics vectors SIMD

* Do not hardcode S.N.Vector IsHardwareAccelerated

Author: radekdoulik

src/mono/System.Private.CoreLib/src/ILLink/ILLink.Substitutions.wasm.xml

@@ -1,8 +1,5 @@
 <linker>
   <assembly fullname="System.Private.CoreLib">
-    <type fullname="System.Numerics.Vector">
-      <method signature="System.Boolean get_IsHardwareAccelerated()" body="stub" value="false" />
-    </type>
     <type fullname="System.Environment">
       <method signature="System.Int32 get_ProcessorCount()" body="stub" value="1" />
     </type>

src/mono/mono/mini/simd-intrinsics.c

@@ -4534,6 +4534,20 @@ arch_emit_simd_intrinsics (const char *class_ns, const char *class_name, MonoCom
 			return emit_vector64_vector128_t (cfg, cmethod, fsig, args);
 	}
 
+	if (!strcmp (class_ns, "System.Numerics") && !strcmp (class_name, "Vector")){
+		return emit_sri_vector (cfg, cmethod, fsig, args);
+	}
+
+	if (!strcmp (class_ns, "System.Numerics") && !strcmp (class_name, "Vector`1")){
+		return emit_vector64_vector128_t (cfg, cmethod, fsig, args);
+	}
+
+	if (!strcmp (class_ns, "System.Numerics")) {
+		//if (!strcmp ("Vector2", class_name) || !strcmp ("Vector4", class_name) || !strcmp ("Vector3", class_name))
+		if (!strcmp ("Vector4", class_name))
+			return emit_vector_2_3_4 (cfg, cmethod, fsig, args);
+	}
+
 	return NULL;
 }
 #else