Excuse regex from doing rustdoc tests atop stage1.

Hopefully final step to get `make check-stage1` working again.

The use of a special-case check for regex here is ugly but is
analogous other similar checks for regex such as the one that landed
in PR rust-lang#13844.

The way this is written, the user will get a reminder that
doc-crate-regex is being skipped whenever their rules attempt to do
the crate documentation tests.  This is deliberate: I want people
running `make check-stage1` to be reminded about which cases are being
skipped.  (But if such echo noise is considered offensive, it can
obviously be removed.)

Author: pnkfelix

mk/tests.mk

@@ -793,8 +793,27 @@ else
 CRATEDOCTESTDEP_$(1)_$(2)_$(3)_$(4) = $$(RSINPUTS_$(4))
 endif
 
+# (Issues #13732, #13983, #14000) The doc for the regex crate includes
+# uses of the `regex!` macro from the regex_macros crate.  There is
+# normally a dependence injected that makes the target's regex depend
+# upon the host's regex_macros (see #13845), but that dependency
+# injection is currently skipped for stage1 as a special case.
+#
+# Therefore, as a further special case, this conditional skips
+# attempting to run the doc tests for the regex crate atop stage1,
+# (since there is no regex_macros crate for the stage1 rustc to load).
+#
+# (Another approach for solving this would be to inject the desired
+# dependence for stage1 as well, by setting things up to generate a
+# regex_macros crate that was compatible with the stage1 rustc and
+# thus re-enable our ability to run this test.)
+ifeq (stage$(1)-crate-$(4),stage1-crate-regex)
+check-stage$(1)-T-$(2)-H-$(3)-doc-crate-$(4)-exec:
+	@$$(call E, skipping doc-crate-$(4) as it uses macros and cannot run at stage$(1))
+else
 check-stage$(1)-T-$(2)-H-$(3)-doc-crate-$(4)-exec: \
 	$$(call TEST_OK_FILE,$(1),$(2),$(3),doc-crate-$(4))
+endif
 
 ifeq ($(2),$$(CFG_BUILD))
 $$(call TEST_OK_FILE,$(1),$(2),$(3),doc-crate-$(4)): $$(CRATEDOCTESTDEP_$(1)_$(2)_$(3)_$(4))