Only pass "-Xgc:CMC" when building for Android.

The runtime always uses CC on host unless CMC is explicitly asked
(http://cs/android-internal/art/runtime/gc/collector/mark_compact.cc;l=212-214;rcl=50fe05b4f4650bc0888d2c218a1bfa476f811ea0).
This CL matches the build system behavior with the runtime.

Bug: 285228269
Test: m test-art-host-gtest-art_dex2oat_tests
Change-Id: I652ad6f754ab8c51283e5fb0e9e1ba644629fa11