arch: Unify declaration of text region

The declaration for the text region '__text_region_start' and
'__text_region_end' should be consistent with the declaration in the
include file 'zephyr/linker/linker-defs.h'.

The local declarations are:
  extern uintptr_t __text_region_start, __text_region_end;
whereas 'linker_defs.h' declares them as:
  extern char __text_region_start[];
  extern char __text_region_end[];

This may result in conflicting types when 'linker_defs.h' is indirectly
included. Hence, remove the local declarations.

Signed-off-by: Christoph Winklhofer <cj.winklhofer@gmail.com>