Consolidate Linux poll implementation

This patch consolidates the poll Linux syscall implementation on
sysdeps/unix/sysv/linux/poll.c.  It basically removes poll from
auto-generation list and add a default implementation that either
call __NR_poll directly (if the kernel headers defines it) or
ppoll adjusting the timeout argument (as the generic implementation).

Checked on i686-linux-gnu, x86_64-linux-gnu, x86_64-linux-gnux32,
aarch64-linux-gnu, arm-linux-gnueabihf, and powerpc64le-linux-gnu.

	* sysdeps/unix/sysv/linux/generic/poll.c: Remove file.
	* sysdeps/unix/sysv/linux/poll.c: New file.
	* sysdeps/unix/sysv/linux/syscalls.list: Remove poll from
	auto-generation list.