rustc: add a --print target-list command

src/test/run-make/print-target-list/Makefile

@@ -0,0 +1,15 @@
+-include ../tools.mk
+
+# Checks that all the targets returned by `rustc --print target-list` are valid
+# target specifications
+# TODO remove the '*ios*' case when rust-lang/rust#29812 is fixed
+all:
+	for target in $(shell $(BARE_RUSTC) --print target-list); do \
+		case $$target in \
+			*ios*) \
+				;; \
+			*) \
+				$(BARE_RUSTC) --target $$target --print sysroot \
+				;; \
+			esac \
+	done