riscv64: fix build breakage

Repro:
do a checkout of aosp-main
$ source build/envsetup.sh
$ lunch aosp_riscv64-userdebug
$ art/tools/buildbot-build.sh --target --installclean

Build failures:
Check failed: !"undefined var" SANITIZER_DEFAULT_VENDOR is not defined

Test: on aosp-main, lunch aosp_riscv64-userdebug, buildbot-build.sh --target
Bug: 300291157
Bug: 283082047
Change-Id: Ieb68dbe950585f1633972beba922390a2f16f6e0