arch-x86: Granularity bit and segment limit

arch-x86: Granularity bit and segment limit

If set, the granularity bit indicates that the segment limit of segment
descriptors shall be interpreted as number of 4K blocks rather than
bytes.

The high part (bit 48 to 51) of segment descriptor limits is only 4 bits
wide while the low part (bit 0 to 15) spans 16 bits.

Change-Id: Ie386224ca815275fdb31498fe68310ed9c62cc87
Reviewed-on: https://gem5-review.googlesource.com/7601
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>