[libc++] Don't alias quick_exit if __ANDROID_API__ < 21

Summary:
quick_exit() and at_quick_exit() were introduced in android NDK 21:
https://android.googlesource.com/platform/prebuilts/ndk/+/dev/platform/sysroot/usr/include/stdlib.h#55

This CL conditions `_LIBCPP_HAS_QUICK_EXIT` on `__ANDROID_API__ >= 21`.  The only place this macro is used is in some using declarations: `using ::quick_exit`, `using ::at_quick_exit`.

Also, add a missing include to sys/cdefs.h which is what defines `__BIONIC__`.

Reviewers: thakis, danalbert, EricWF

Reviewed By: danalbert

Subscribers: srhines, krytarowski

Differential Revision: https://reviews.llvm.org/D39479

llvm-svn: 317124