Set incremental or codegen-units, not both

This CL changes the handling of Rust flags to only pass one of either
incremental or codegen-units.  This is necessary because incremental
implies a codegen-units value of 128 and it may be overridden by the
separate codegen-units argument.

Test: TH
Change-Id: Ia474e4f9fc071d5054cb479ec9de550fa886677e
diff --git a/rust/builder.go b/rust/builder.go
index bea8ff7..fe2d03a 100644
--- a/rust/builder.go
+++ b/rust/builder.go
@@ -278,7 +278,9 @@
 	if ctx.Config().IsEnvTrue("SOONG_RUSTC_INCREMENTAL") {
 		incrementalPath := android.PathForOutput(ctx, "rustc").String()
 
-		rustcFlags = append(rustcFlags, "-Cincremental="+incrementalPath)
+		rustcFlags = append(rustcFlags, "-C incremental="+incrementalPath)
+	} else {
+		rustcFlags = append(rustcFlags, "-C codegen-units=1")
 	}
 
 	// Disallow experimental features