Add $ANDROID_BUILD_TOP to GDB source search paths

Without this change, GDB doesn't display the sources e.g. upon the
`list` command or when run with `--tui` option.

Test: art/test.py -v -j56 --host --64 --optimizing \
      --ndebuggable --gdb -t 071-dexfile-map-clean \
      --run-test
Test: art/test.py -v -j56 --host --64 --optimizing \
      --ndebuggable --gdb-dex2oat -t 071-dexfile-map-clean \
      --run-test
Change-Id: If7e43f3d0515333e5e9713f56cd62960e7e9a0c6
1 file changed