Disable dexpreopt for aosp_riscv64

dex2oat isn't supported for riscv64 yet, disable dexpreopt.  For
aosp_riscv64-user and -userdebug builds it is also necessary to
relax the requirement that dexpreopt is enabled.

Test: trehugger
Change-Id: Ib84f1b40675783d286f4ff89d3c1daa9c7be710a