Revert "Disable dexpreopt for aosp_riscv64"

This reverts commit e325f61525ccf56ef4e4ae60b56d9626de88f5f1.

Reason for revert: DroidMonitor-triggered revert due to breakage in some branches.

Bug: 270958097
Change-Id: I2c3bb568c7f3b0dc7a364ea74b91574966102ff3
diff --git a/core/board_config.mk b/core/board_config.mk
index bffaf2a..70c91a8 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
 
 # ###############################################################