Revert "Use prebuilt NDK libc++ for rv64."

Revert submission 2676418-hack-rv64-libc++

Reason for revert: build break

Reverted changes: /q/submissionid:2676418-hack-rv64-libc%252B%252B

Change-Id: Idd7a5fd3b1687e2a31aa3c036d5d851fb5bbe283
Bug: 293214158
Bug: 293214503
Bug: 293214158
Bug: 293219471

cc/stl.go

@@ -80,7 +80,8 @@ func (stl *stl) begin(ctx BaseModuleContext) {
 			return ""
 		}
 		s = deduplicateStlInput(s)
-		if ctx.useSdk() && ctx.Device() {
+		archHasNDKStl := ctx.Arch().ArchType != android.Riscv64
+		if ctx.useSdk() && ctx.Device() && archHasNDKStl {
 			switch s {
 			case "", "system":
 				return "ndk_system"