HACK: use arm64 apex prebuilts for riscv64

There are no riscv64 apex prebuilts, which breaks the build when
provenance_metadata singelton tries to run a tool on all the input
apexes that don't exist.  Hack it to provide the arm64 apex for now
if no riscv64 apex is specified, which will allow the build to pass.

Test: lunch aosp_riscv64-userdebug && m droid
Change-Id: Ic39936539803615ea4b7f817daf7b59ab7c40e7e
diff --git a/apex/prebuilt.go b/apex/prebuilt.go
index 70308c8..39446a1 100644
--- a/apex/prebuilt.go
+++ b/apex/prebuilt.go
@@ -532,6 +532,10 @@
 		src = String(p.Arch.Arm64.Src)
 	case android.Riscv64:
 		src = String(p.Arch.Riscv64.Src)
+		// HACK: fall back to arm64 prebuilts, the riscv64 ones don't exist yet.
+		if src == "" {
+			src = String(p.Arch.Arm64.Src)
+		}
 	case android.X86:
 		src = String(p.Arch.X86.Src)
 	case android.X86_64: