Fix DllImport of IdnToAscii & IdnToUnicode (#8666)

Fix an issue found in OneCoreUAP testing. According to MSDN,
the official exporting DLL for IdnToAccii and IdnToUnicode
is normaliz.dll, not kernel32.dll. While most Windows SKUs
export these functions from both normaliz.dll and kernel32.dll,
recent tests revealed that some Windows SKUs export them from
normaliz.dll only.

src/mscorlib/src/System/Globalization/IdnMapping.cs

@@ -1158,7 +1158,7 @@ MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
 
 
  [SuppressUnmanagedCodeSecurityAttribute()]
- [DllImport("kernel32.dll", CharSet=CharSet.Unicode, SetLastError=true)]
+ [DllImport("normaliz.dll", CharSet=CharSet.Unicode, SetLastError=true)]
  private static extern int IdnToAscii(
  uint dwFlags, 
  [InAttribute()]
@@ -1171,7 +1171,7 @@ private static extern int IdnToAscii(
  int cchASCIIChar);
 
  [SuppressUnmanagedCodeSecurityAttribute()]
- [DllImport("kernel32.dll", CharSet=CharSet.Unicode, SetLastError=true)]
+ [DllImport("normaliz.dll", CharSet=CharSet.Unicode, SetLastError=true)]
  private static extern int IdnToUnicode(
  uint dwFlags, 
  [InAttribute()]