arm arch: Replace use of TOSTR with STRINGIFY

One of the ARM architure files, defined since long ago
TOSTR having the exact same purpose as Zephyr's STRINGIFY.

Remove the use of this macro in the tree
(only used in another ARM header file) in favour
of STRINGIFY.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>