Do not call `gensym' too early when loading a dump file.

Do not call `gensym' too early when loading a dump file.

This happened when subr.eln was not the first native compilation unit
to be loaded. register_native_comp_unit() is called when loading a
native compilation unit and that in turn used to call `gensym', which
was not loaded yet. This led to a SIGSEGV.

* src/comp.c (register_native_comp_unit): Replace the call to `gensym'
with an ad-hoc counter.