x64: enable A20 gate very early

Without this, only even megabytes are accessible, and accesses to odd megabytes
hit the even megabytes.  For example address 0x312345 is aliased to address
0x212345.

Enable the A20 gate to prevent this.

Fixes boot on VMware.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>