multiple fixes

- avoid choking in morestack on i586,
 - fixup dependencies
 - rename rust-cross to internal-rust-cross (in the hope the people are
   less likely to try to inherit it when they shouldn't)
 - enable-local-rust for -target and -cross builds

classes/internal-rust-native.bbclass

@@ -0,0 +1,5 @@
+inherit native
+PN = "rust-native"
+
+USE_LOCAL_NATIVE_RUST ??= "0"
+USE_LOCAL_RUST = "${base_conditional('USE_LOCAL_NATIVE_RUST', '0', '0', '1')}"