Move backup related stuff to its own Makefile.

This will prevent Makefile errors from affecting backups. This happens when
different branches include Makefile helpers other branches don't have. This
should be more reliable.

Makefile.backup

@@ -0,0 +1,17 @@
+# Build commands for backing up and restoring the Rust build.
+# Because Rust takes a very long time to build and changes infrequently,
+# the bots want to reuse it between builds. They can do so by running
+# `make backup-rust` and `make restore-rust`.
+#
+# backup-rust is run before the build directory is cleaned. if it fails, that's ok.
+#
+# restore-rust is run after configure and before invoking make. if it fails, we must abort.
+
+include config.mk
+
+.PHONY: backup-rust restore-rust
+backup-rust:
+	-mv $(CFG_BUILD_DIR)src/compiler/rust ../$(CFG_TARGET_TRIPLES)
+
+restore-rust:
+	if [ -d ../$(CFG_TARGET_TRIPLES) ]; then rm -rf $(CFG_BUILD_DIR)src/compiler/rust; mv ../$(CFG_TARGET_TRIPLES) $(CFG_BUILD_DIR)src/compiler/rust; fi

Makefile.in

@@ -363,21 +363,13 @@ package:
 
 endif
 
-# Build commands for backing up and restoring the Rust build.
+# Backup targets. These axist until build slaves are using Makefile.backup directly.
-# Because Rust takes a very long time to build and changes infrequently,
-# the bots want to reuse it between builds. They can do so by running
-# `make backup-rust` and `make restore-rust`.
-#
-# backup-rust is run before the build directory is cleaned. if it fails, that's ok.
-#
-# restore-rust is run after configure and before invoking make. if it fails, we must abort.
 
 .PHONY: backup-rust restore-rust
 backup-rust:
-	-mv $(B)src/compiler/rust ../$(CFG_TARGET_TRIPLES)
+	$(MAKE) -f $(S)Makefile.backup $@
 
 restore-rust:
-	if [ -d ../$(CFG_TARGET_TRIPLES) ]; then rm -rf $(B)src/compiler/rust; mv ../$(CFG_TARGET_TRIPLES) $(B)src/compiler/rust; fi
+	$(MAKE) -f $(S)Makefile.backup $@
 
 .NOTPARALLEL:
-