x86: access: Add test for 5 level paging mode

Provide paging mode switching logic to run access test in 5
level paging mode if LA57 is detected. Qemu parameter +la57
should be used to expose this feature, for example:
./x86-run ./x86/access.flat -cpu qemu64,+la57

Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
Message-Id: <1503662251-13895-1-git-send-email-yu.c.zhang@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>