Clean up duplication in APM gn file.
Clean up duplication in APM gn file.

I realized I could use configs to fix some duplication that I
partially introduced.

Verified APM_DEBUG_DUMP is set appropriately by looking at the
compiler command line.

Bug: webrtc:6828
Change-Id: Ia990e2721546d65639567cd3ab788439e328c5da
Reviewed-on: https://webrtc-review.googlesource.com/34642
Reviewed-by: Per Åhgren <3616c2c8a2c30a769c5419f89b4a810ea6e2f079@webrtc.org>
Commit-Queue: Patrik Höglund <b486136ee68458b09c92c86c39f451c87d6ef6a1@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#21349}
ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc