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