Only clear system_shared_libs when building for bionic

arm-optimized-routines is included in bionic, so it sets
system_shared_libs to empty to avoid a circular dependency.
Clear system_shared_libs only when compiling for bionic so that
the host tests can compile against musl.

Bug: 190084016
Test: m USE_HOST_MUSL=true host-native
Change-Id: I6915c45bb271683f04c2a5cbf9cbcfa2e332faba