Configurations/10-main.conf: add -fno-common back to darwin-ppc-cc.

-fno-common was removed for all Darwin targets in
0c873419

 with rationale "it's either
'ranlib -c' or '-fno-common'." However, it's still absolutely required
in 32-bit darwin-ppc-cc. And when trying things out I didn't quite
see why it was formulated as one-or-another choice, as 'ranlib -c'
shouldn't [and doesn't] have problems with object modules without
commons. [Well, to be frank, I didn't manage to reproduce the problem
the modification was meaning to resolve either...]

Reviewed-by: Rich Salz <rsalz@openssl.org>