Fixed the TAGS.rustc.emacs and TAGS.rustc.vi make targets.

(They were added to `ctags.mk` in PR rust-lang#33256, but I guess I must have
 only tested running `make TAGS.emacs TAGS.rustc.emacs` and not `make
 TAGS.rustc.emacs` on its own.)

Makefile.in

@@ -277,7 +277,9 @@ endif
 
 # CTAGS building
 ifneq ($(strip $(findstring TAGS.emacs,$(MAKECMDGOALS)) \
-               $(findstring TAGS.vi,$(MAKECMDGOALS))),)
+               $(findstring TAGS.vi,$(MAKECMDGOALS)) \
+               $(findstring TAGS.rustc.emacs,$(MAKECMDGOALS)) \
+               $(findstring TAGS.rustc.vi,$(MAKECMDGOALS))),)
   CFG_INFO := $(info cfg: including ctags rules)
   include $(CFG_SRC_DIR)mk/ctags.mk
 endif