[lldb] Fix some tests failing with gmodules after change to stdlib.h

Commit 82b47b29 changes the way the stdlib.h
header is structured which seems to cause strange lookup failures in the modules
build. This updates a few failing tests so that they pass with the new
behavior of stdlib.h.

See the discussion in https://reviews.llvm.org/rG82b47b2978405f802a33b00d046e6f18ef6a47be