Remove unnecessary 10.8-specific build rules

We no longer need gcc for host builds, since those all run through clang. This
header include, however, triggers errors about SSE intrinsics by replacing
the more relevant include dirs that we should be using.

Change-Id: I26a949f0109de8e6e2d1f09cb8127be927549cc4
Merge "Remove unnecessary 10.8-specific build rules"