X86: Enable x86_64 vsyscall support

X86: Enable x86_64 vsyscall support

64-bit vsyscall is different than 32-bit.
There are only two syscalls, time and gettimeofday.
On a real system, there is complicated code that implements these
without entering the kernel.  That would be complicated to implement in m5.
Instead we just place code that calls the regular syscalls (this is how
tools such as valgrind handle this case).

This is needed for the perlbmk spec2k benchmark.