[corlib] Ensure FileIOPermission ctor is not called on mobile

[corlib] Ensure FileIOPermission ctor is not called on mobile

After https://github.com/mono/mono/pull/3452 was merged, the LinkSdkRegressionTest.SecurityDeclaration
test in XI started failing:

```
	[FAIL] LinkSdkRegressionTest.SecurityDeclaration :   FileIOPermissionAccess
  Expected: null
  But was:  <System.Security.Permissions.FileIOPermissionAccess>

		  at LinkSdk.LinkSdkRegressionTest.SecurityDeclaration () [0x00033] in /Users/builder/data/lanes/1411/2a96404f/source/xamarin-macios/tests/linker-ios/link sdk/LinkSdkRegressionTest.cs:990
		  at (wrapper managed-to-native) System.Reflection.MonoMethod:InternalInvoke (System.Reflection.MonoMethod,object,object[],System.Exception&)
		  at System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00038] in /Users/builder/data/lanes/1411/2a96404f/source/xamarin-macios/_ios-build/Library/Frameworks/Xamarin.iOS.framework/Versions/git/src/mono/mcs/class/corlib/System.Reflection/MonoMethod.cs:305
```

Due to the added call in the PR the linker preserved FileIOPermissionAccess enum,
which is apparently something we want to avoid.

Since according to https://github.com/mono/mono/pull/3452#discussion_r76384945 the only point of calling the
FileIOPermission is validating that the path doesn't contain invalid characters (mostly an issue on Windows),
I decided to just copy those checks as it's way cleaner.