Browsing the source for "cc" in v6 and v7, if invoked with -2 would
replace crt0.o with crt2.o. If the -2 were followed by another character
(probably intended to be -20), it would use crt20.o and use -l2 instead
of -lc.
These options seem to be undocumented, and I can't find any source code
of these libraries or indication as to what the purpose was.