Add riscv64-linux-android support

Add barebones riscv64-linux-android support.  This should be enough
to add riscv64-specific entries to Android.bp files, but can't
actually compile anything until there are riscv64 toolchains.

Test: arch_test.go
Change-Id: I0dcc7e797d9352dd38243be908a7f19004ff3db1
diff --git a/rust/config/Android.bp b/rust/config/Android.bp
index be73d69..79ea7a1 100644
--- a/rust/config/Android.bp
+++ b/rust/config/Android.bp
@@ -15,6 +15,7 @@
         "arm64_device.go",
         "global.go",
         "lints.go",
+        "riscv64_device.go",
         "toolchain.go",
         "darwin_host.go",
         "x86_linux_bionic_host.go",