boards/qemu_x86: fix qemu memory map

The desired memory map is to have the 0 - 4K page non-present
to catch NULL pointer dereferences,
from 4K - 4MB for the program text (RO, Execute),
ROM (RO, No Execute), and 4MB-8MB for system RAM.

This patch cut text size by 4 KB which allow to meet above
requirements.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>