depend-cleanup.sh: Fix overzealous syscall.S cleanup on non-amd64

Although there is no longer a syscall.S in the source tree, the
generated file is still called syscall.S, so the syscall\.S regex still
matches the .depend file. Previously it looked like:

syscall.o \
  /path/to/src/lib/libc/MACHINE/sys/syscall.S \
  ...

or

syscall.o \
  /path/to/src/lib/libsys/MACHINE/syscall.S \
  ...

Now it looks like:

syscall.o: syscall.S \
  ...

Thus, match on the file having a directory specified, to distinguish it from
the generated case where it's adjacent in the OBJDIR. Note the .* is needed due
to the use of egrep -w.

Fixes: 3a4b04e82df6 ("depend-cleanup.sh: Simplify handling for stale syscall.S")