[Analysis] Only use _unlocked stdio functions on linux
The existing comment said that the functions were available only
on GNU/Linux (and on certain Android versions), but only checked
T.isGNUEnvironment() which also is true on MinGW (for arch-windows-gnu
triplets), which doesn't have such functions.
Existing checks in the initialize function in TargetLibraryInfo.cpp
also use only T.isOSLinux() to check for glibc features.
This fixes use of stdio on MinGW.
Differential Revision: https://reviews.llvm.org/D47002