cstdio: limit gets to CRT versions below 14

Microsoft removed gets from the CRT in Visual Studio 2015 onwards [1].
Attempting to reference it when targeting CRT versions 14 and above will cause
compile errors.

[1] https://msdn.microsoft.com/en-us/library/2029ea5f.aspx

Patch by Shoaib Meenai!

llvm-svn: 280417