Add explicit arch-specific modules.c rules to keep the build from
generating an incorrect common/modules.c. Otherwise the pattern
rules would cascade such that it'd look for $arch/modules.o which
turned into common/modules.c which triggered the gen rule.
My local testing of this code didn't catch this bug because of how
Automake manages .Po (dependency files) in incremental builds -- it
was adding extra rules that override the pattern rules which caused
the build to generate correct modules.c files. But when building
from a cold cache, the pattern rules would force common/modules.c to
be used leading to crashes at runtime.
