syscall: Fix generator for Linux syscalls.

syscall: Fix generator for Linux syscalls.

In golang.org/cl/14449 the `getdents' system call got changed to use
_SYS_getdents as a layer of indirection instead of SYS_GETDENTS64 for
compatibility with mips64, but this broke mksyscall.pl, which then
died with with:
  syscall_linux.go:840: malformed //sys declaration

Change-Id: Icb61965d8730f6e81f9fb0fa28c7bab635470f09
Reviewed-on: https://go-review.googlesource.com/20051
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>