Link libsigchain.a instead of sigchain.o

  Relative paths may sometimes lead to failed builds
  if sources are built in particular order.

  Also it is possible to export symbols from static lib
  with LOCAL_WHOLE_STATIC_LIBRARIES variable.

(cherry picked from commit e49a4f6d)

Bug: 18701723
Change-Id: I6d62594c2e84020d67254747c1fd465e1144920e