Define N_SYMSIZE in case the system does not do it
Fixes build failures on some non-Linux ports.
Based-on-patch-by: Petr Salinger <Petr.Salinger@seznam.cz>
Use N_SYMSIZE instead of directly accessing ‘struct exec’
On Linux alpha ‘struct exec’ does not have an a_syms member, and the
N_SYMSIZE is mapped to 0.