Don't pack relocations in libc.so

Ic9125cc1bc4c9ba9eb20d030de72e3ce1fb86fa6 disabled relocation packing
in libc.so, but the flag was dropped when we switched to Android.bp
in Ib0ba2d28bff88483b505426ba61606da314e03ab.

Bug: 20645321
Bug: 20655855
Bug: 32750616
Test: builds
Test: inspect out/build-aosp_angler.ninja
Test: run app that crashes with packed libc.so
Change-Id: Ib0630f2e342afa543bb88303fec8f9695ac583e9