nptl: Fix cleanups for stack grows up [BZ# 28899]

_STACK_GROWS_DOWN is defined to 0 when the stack grows up.  The
code in unwind.c used `#ifdef _STACK_GROWS_DOWN' to selct the
stack grows down define for FRAME_LEFT.  As a result, the
_STACK_GROWS_DOWN define was always selected and cleanups were
incorrectly sequenced when the stack grows up.

(cherry picked from commit 2bbc694d)