stdlib: fix build for LLP64 targets

When building the stdlib for Windows x86_64, we would see the following error:

  swift/stdlib/public/core/RuntimeFunctionCounters.swift:95:19: error: '(UnsafeRawPointer, Int64) -> Void' is not representable in Objective-C, so it cannot be used with '@convention(c)'
     @convention(c) (_ object: UnsafeRawPointer, _ functionId: Int64) -> Void
                    ^

This is caused by `Int64` not being mapped as on Windows x86_64, `CLong`
is mapped to `Int32` and `CLongLong` is mapped to `Int`.  This causes
the `Int64` to fail to be reverse-mapped to a C type causing the FFI
construction failure.

Author: compnerd

stdlib/public/core/RuntimeFunctionCounters.swift

@@ -91,8 +91,13 @@ internal func _collectAllReferencesInsideObjectImpl(
 // of runtime function counters.
 public // @testable
 struct _RuntimeFunctionCounters {
+#if os(Windows) && arch(x86_64)
   public typealias RuntimeFunctionCountersUpdateHandler =
-   @convention(c) (_ object: UnsafeRawPointer, _ functionId: Int64) -> Void
+    @convention(c) (_ object: UnsafeRawPointer, _ functionId: Int) -> Void
+#else
+  public typealias RuntimeFunctionCountersUpdateHandler =
+    @convention(c) (_ object: UnsafeRawPointer, _ functionId: Int64) -> Void
+#endif
 
   public static let runtimeFunctionNames =
     getRuntimeFunctionNames()