lib{c,sys}: fix incremental builds

I removed lib/libsys/{aarch64,arm,riscv}/syscall.S in favor of an
idential generated version. We need to clean out the .ddepend files to
ensure the generated version is actually generated.

The guard here is technically too strict, but should be fine in practice
and I've verified both the breakage and fix on an armv7 build.

Reported by: imp
Fixes: e6ffc7669a56 Remove pointless MD syscall(2)
Fixes: 0ee0ae237324 Remove pointless MD syscall(2)
Fixes: 7b3836c28188 Remove pointless MD syscall(2)