x86: implement la57 paging mode

The new paging more is extension of IA32e mode with more additional page
table level.

It brings support of 57-bit vitrual address space (128PB) and 52-bit
physical address space (4PB).

The structure of new page table level is identical to pml4.

The feature is enumerated with CPUID.(EAX=07H, ECX=0):ECX[bit 16].

CR4.LA57[bit 12] need to be set when pageing enables to activate 5-level
paging mode.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Message-Id: <20161215001305.146807-1-kirill.shutemov@linux.intel.com>
[Drop changes to target-i386/translate.c. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>