Undef EPOLL_CLOEXEC for pre-L.

Some third-party code uses the existence of EPOLL_CLOEXEC to detect
the availability of epoll_create1. This is not correct, since having
up-to-date UAPI headers says nothing about the C library, but for the
time being we don't want to harm adoption to the unified headers.
We'll undef EPOLL_CLOEXEC if we don't have epoll_create1 for the time
being, and maybe revisit this later.

Test: make checkbuild
Bug: https://github.com/android-ndk/ndk/issues/302
Bug: https://github.com/android-ndk/ndk/issues/394
Change-Id: I8ee9ce62768fb174070ec51d114f477389befc4a