x86_64/vsyscall: Move all of the gate_area code to vsyscall_64.c

This code exists for the sole purpose of making the vsyscall
page look sort of like real userspace memory.  Move it so that
it lives with the rest of the vsyscall code.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Link: http://lkml.kernel.org/r/a7ee266773671a05f00b7175ca65a0dd812d2e4b.1411494540.git.luto@amacapital.net


Signed-off-by: Ingo Molnar <mingo@kernel.org>