Generate NDK stubs for riscv64 too.

These will only be provisional, but there are enough different groups
that want *something* to make a start on porting that it's probably
time...

Ignore missing dependencies in build-ndk-prebuilts.sh for now, because
various pieces (that the NDK doesn't care about) are still missing.

Bug: http://b/273792258
Test: build/soong/scripts/build-ndk-prebuilts.sh
Change-Id: I39202a953a73dc449a10a887d497d09079c43402
diff --git a/scripts/build-ndk-prebuilts.sh b/scripts/build-ndk-prebuilts.sh
index b57963b..964a866 100755
--- a/scripts/build-ndk-prebuilts.sh
+++ b/scripts/build-ndk-prebuilts.sh
@@ -19,7 +19,10 @@
     exit 1
 fi
 
-TARGET_PRODUCT=ndk build/soong/soong_ui.bash --make-mode --soong-only ${OUT_DIR}/soong/ndk.timestamp
+# TODO: remove this when all the riscv64 dependencies exist (currently blocked by
+# http://b/273792258).
+ALLOW_MISSING_DEPENDENCIES=true \
+    TARGET_PRODUCT=ndk build/soong/soong_ui.bash --make-mode --soong-only ${OUT_DIR}/soong/ndk.timestamp
 
 if [ -n "${DIST_DIR}" ]; then
     mkdir -p ${DIST_DIR} || true