x86: Fixed incorrect segment base address addition in 64-bits mode

According to the Intel manual
"Intel® 64 and IA-32 Architectures Software Developer’s Manual
Volume 3", "3.4.4 Segment Loading Instructions in IA-32e Mode":

"When in compatibility mode, FS and GS overrides operate as defined by
32-bit mode behavior regardless of the value loaded into the upper 32
linear-address bits of the hidden descriptor register base field.
Compatibility mode ignores the upper 32 bits when calculating an effective address."

However, the code misses the 64-bit mode case, where an instruction with
address and segment size override would be translated incorrectly. For example,
inc dword ptr gs:260h[ebx*4] gets incorrectly translated to:

(uint32_t)(gs.base + ebx * 4 + 0x260)
instead of
gs.base + (uint32_t)(ebx * 4 + 0x260)

Signed-off-by: Vitaly Chipounov <vitaly.chipounov@epfl.ch>
Reviewed-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>