Use libcompiler-rt_extras with NDK builds.

We want to build things in CTS (things built against the NDK) with
integer overflow checks. Some projects in the tree also make explicit
calls to the overflow checked builtins, and those projects need to be
built with the NDK (external/dng_sdk is the specific example that
brought this up).

Test: make checkbuild tests
Bug: None
Change-Id: Iab27dd0d931f723140de242049fa72f3b1ce0fb8