Makefile: use shell to calculate map_size

The error message "bc: command not found" is easily missed since the
build continues.
bc is not a part of coreutils or base-devel. POSIX sh can also do the
calculation.

Signed-off-by: Leon M. Busch-George <leon@georgemail.eu>
Reviewed-by: Dragan Simic <dsimic@manjaro.org>