Set ALLOW_MISSING_DEPENDENCIES=true in generic_riscv64/BoardConfig.mk

Building for riscv64 without ALLOW_MISSING_DEPENDENCIES=true fails
on prebuilt modules that are missing riscv64 files.  Set
ALLOW_MISSING_DEPENDENCIES=true in BoardConfig.mk for now to avoid
always having to set it on the command line.

Test: lunch aosp_riscv64-userdebug && m ndk_sysroot
Change-Id: I6b7dcf684b59d073cb6e257717946d551aefdb44