runtime: enable shims for Float16 on Windows (x86)

When building for Windows x86, we do not have `__i686__` defined, but do
have `__i386__` defined.  Ensure that the routines are included for the
x86 Windows target.

Author: compnerd

stdlib/public/runtime/Float16Support.cpp

@@ -30,7 +30,7 @@
 // Android NDK <r21 do not provide `__aeabi_d2h` in the compiler runtime,
 // provide shims in that case.
 #if (defined(__ANDROID__) && defined(__ARM_ARCH_7A__) && defined(__ARM_EABI__)) || \
-  ((defined(__i686__) || defined(__x86_64__)) && !defined(__APPLE__))
+  ((defined(__i386__) || defined(__i686__) || defined(__x86_64__)) && !defined(__APPLE__))
 
 #include "../SwiftShims/Visibility.h"