arch: arm: Fix missing include for sys_define_gpr_with_alias

The sys_define_gpr_with_alias macro was defined by arch.h,
but was used by the two exception.h files that did not include it.

Attempting to include arch.h from exception.h would resolve
in a recursive include, so the macro was simply moved to the
files that used it.

An alternative could have been to define a new file that defined it,
that could have been included by exception.h

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>