rpcgen: make compiler arglist allocation dynamic

Limit argmax to an absurdly large value prevent overflow (no overflow
possible on FreeBSD due to ARG_MAX).

In CheriBSD we exceed the 19 non-NULL arguments in the static array. Add
a simple size doubling allocator and increase the default to 32.

GC remnants of support for fixed arguments.

Reviewed by: archardson (prior version), James Clarke (prior version)
MFC after: 1 week
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D21971