mem_safe: prevent writing to .text in XIP systems

The sys_mem_safe_write_to_text_section() now always fails in XIP
systems, since their .text section is in ROM.

Change-Id: Ie47a5dbf5f75a4bfe8e9fc9852d0037a3546aae8
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>