Switch libhevc over to using Clang.

Bug: http://b/25132373

This reverts a previous patch that set LOCAL_CLANG := false, and also
redefines "GCC_ENABLE" to 0, since those intrinsics weren't supported in
Android's gcc (even though we aren't using AVX anywhere).

Change-Id: Iae686da20f178e87ea0ffea29551adf4414722af
2 files changed