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
diff --git a/core/board_config.mk b/core/board_config.mk
index 70c91a8..bffaf2a 100644
--- a/core/board_config.mk
+++ b/core/board_config.mk
@@ -203,7 +203,7 @@
 
 # Conditional to building on linux, as dex2oat currently does not work on darwin.
 ifeq ($(HOST_OS),linux)
-  WITH_DEXPREOPT := true
+  WITH_DEXPREOPT ?= true
 endif
 
 # ###############################################################