[GCC] Fix some configuration flags for Android. Add --enable_libatomic_ifuncs to control whether or not to use IFUNCS in libatomic; fix the testing for LIBCXX_GTHREADS. BUG=chromium:608771 TEST=Successfully created trybot images for daisy, x86-alex, peppy and oak. Ran correctness & performance autotests. Successfully ran with chromiumos-sdk trybot. Change-Id: I782c39a641cf81731557e59e681f924421a3a245 Reviewed-on: https://chromium-review.googlesource.com/341960 Commit-Ready: Caroline Tice <[email protected]> Tested-by: Caroline Tice <[email protected]> Reviewed-by: Yunlian Jiang <[email protected]>