art: use -O1 to compile tests with clang for arm64

arm64 clang fails to compile art/runtime/arch/stub_test.cc with:
art/runtime/arch/stub_test.cc:178:9: error: inline assembly requires
more registers than available
        "sub sp, sp, #64\n\t"
        ^

Pass -O1 to all art tests compiled with clang for arm64.

Bug: 26275713
Change-Id: Ib1a1c309aef73a2d33d37d720e5e11b8fe7a248e