Don't expect depfile from .s files

.s files (unlike .S files) aren't run through the preprocessor, so -M*
doesn't actually write out a depfile.

Since our ninja is now going to be verifying that the depfile is created
(https://android-review.googlesource.com/861510), don't specify a
depfile for .s files.

Bug: 121058584
Test: apply https://android-review.googlesource.com/861510
Test: cd external/libavc; mma
Change-Id: I1697aa020c63639317c8f4771147026601ae72fc