Statically link LLVM libraries into backtrace_test

Bug: http://b/28866258

Remove dependency of backtrace_test on libLLVM.  Also clean up
specification for libbacktrace_offline module.  Since it is a static
library now, dependencies are only needed to automatically include
necessary headers.

Test: 32-bit and 64-bit backtrace_test passes on host and on Angler.
      mma in system/extras/simpleperf, cts/tests/tests/simpleperf
      m test-art-host

Change-Id: I76b0db14d067fa7b140a99edd237370ac04fe873