arm arch: Remove definition of CONCAT and TOSTR

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

This arm header file is included thru dependencies into
almost all code built for ARM, which leads to these
macros being usable everywhere.

This can easily make developers belive the macros
are provided by Zephyr itself, and use them, leading
to code which is not portable between architectures.

Remove this macros definitions from the architecture
headers.

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