[flang] Disable -Wmaybe-uninitialized for GCC

According to llvm/cmake/config-ix.cmake, gcc's potentially uninitialized
use analysis has lots of false positives. We are encountering one in
flang/lib/Lower/CharacterExpr.cpp.

That warning is disabled for gcc in in-tree builds; this does the same
thing for out-of-tree builds.

Differential Revision: https://reviews.llvm.org/D85694