Fix MIPS n64 readahead (bug 21026).

As noted in bug 20126, MIPS n64 uses an incorrect implementation of
readahead intended for 32-bit systems.  This patch adds a
syscalls.list entry to fix this.  An updated version of the
consolidation patch
<https://sourceware.org/ml/libc-alpha/2016-09/msg00527.html> could
remove this syscalls.list entry again.

Tested with compilation (only) for mips64; the nature of the syscall
doesn't allow for a glibc test to detect this issue.

	[BZ #21026]
	* sysdeps/unix/sysv/linux/mips/mips64/n64/syscalls.list
	(readahead): New syscall entry.