Append whole_static_libs deps from .a files instead of the list of

objects.

Necessary to make whole_static_libs work with
cc_prebuilt_library_static since it doesn't propagate the list of
object files.

Test: Build & boot
Test: m libsigchain && \
  ar t out/soong/.intermediates/art/sigchainlib/libsigchain/android_arm64_armv8-a_cortex-a73_static/libsigchain.a
  (Check that the list is sigchain.o followed by async_safe_log.o, both
  in a normal build and in one where async_safe is a prebuilt static
  lib.)
Bug: 154248570
Change-Id: Iaada8490ce713c13804b5771ad606f4a27e72a2f