Fix ifdef for RawArrayData in NativeAOT Runtime.Base. (#109494)

The `BIT64` preprocessor symbol is not defined nor used anywhere in this
repository. This means that if you were to compile a Runtime.Base assembly
for 64-bit, helpers like `RhpLdelemaRef` would calculate an incorrect address.

This PR fixes the problem by using the `TARGET_64BIT` preprossor directive
instead. Just like the System.Private.CoreLib version of this class.

src/coreclr/nativeaot/Runtime.Base/src/System/Array.cs

@@ -31,7 +31,7 @@ internal class Array<T> : Array
     internal class RawArrayData
     {
         public uint Length; // Array._numComponents padded to IntPtr
-#if BIT64
+#if TARGET_64BIT
         public uint Padding;
 #endif
         public byte Data;