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: lunch aosp_riscv64-userdebug && m
Change-Id: I3b66304cedf55f734f8aa1d3ad3ed594a7b9a122
diff --git a/core/board_config.mk b/core/board_config.mk
index b1b7b81..16acdde 100644
--- a/core/board_config.mk
+++ b/core/board_config.mk
@@ -204,7 +204,10 @@
 
 # Conditional to building on linux, as dex2oat currently does not work on darwin.
 ifeq ($(HOST_OS),linux)
-  WITH_DEXPREOPT := true
+  # TODO(riscv64) add compiler support and enable dexpreopt on RISC-V.
+  ifneq ($(TARGET_ARCH),riscv64)
+    WITH_DEXPREOPT := true
+  endif
 endif
 
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