libgloss: document & localize custom multi-do rule

The multi-build.in file in libgloss duplicates common multilib logic
in the root source tree.  Document it a bit, and rename the rule so
it doesn't clash with the common multi-do rule.  This will let us use
them in the same makefile so we can merge aarch64/ & arm/ up (as the
only targets that use this local multi-build.in atm).

libgloss/aarch64/Makefile.in

@@ -152,7 +152,7 @@ $(RDIMON_BSP): $(RDIMON_OBJS)
 	${RANLIB} $@
 
 # Multilib support.
-.PHONY:  multi-do all-multi clean-multi install-multi
+.PHONY:  libgloss-multi-do all-multi clean-multi install-multi
 
 all-recursive: all-multi
 clean-recursive: clean-multi
@@ -160,13 +160,13 @@ install-recursive: install-multi
 
 # The $(MAKE) comments below are to enable parallel building.
 all-multi:
-	$(MAKE) $(AM_MAKEFLAGS) DO=all multi-do
+	$(MAKE) $(AM_MAKEFLAGS) DO=all libgloss-multi-do
 
 clean-multi:
-	$(MAKE) $(AM_MAKEFLAGS) DO=clean multi-do
+	$(MAKE) $(AM_MAKEFLAGS) DO=clean libgloss-multi-do
 
 install-multi:
-	$(MAKE) $(AM_MAKEFLAGS) DO=install multi-do
+	$(MAKE) $(AM_MAKEFLAGS) DO=install libgloss-multi-do
 
 clean mostlyclean:
 	rm -f a.out core *.i *.o *-test *.srec *.dis *.x $(SIM_BSP) ${RDIMON_SCRIPTS}

libgloss/arm/Makefile.in

@@ -175,7 +175,7 @@ $(RDIMON_BSP): $(RDIMON_OBJS)
 	${RANLIB} $@
 
 # Multilib support.
-.PHONY:  multi-do all-multi clean-multi install-multi
+.PHONY:  libgloss-multi-do all-multi clean-multi install-multi
 
 all-recursive: all-multi
 clean-recursive: clean-multi
@@ -183,13 +183,13 @@ install-recursive: install-multi
 
 # The $(MAKE) comments below are to enable parallel building.
 all-multi:
-	$(MAKE) $(AM_MAKEFLAGS) DO=all multi-do
+	$(MAKE) $(AM_MAKEFLAGS) DO=all libgloss-multi-do
 
 clean-multi:
-	$(MAKE) $(AM_MAKEFLAGS) DO=clean multi-do
+	$(MAKE) $(AM_MAKEFLAGS) DO=clean libgloss-multi-do
 
 install-multi:
-	$(MAKE) $(AM_MAKEFLAGS) DO=install multi-do
+	$(MAKE) $(AM_MAKEFLAGS) DO=install libgloss-multi-do
 
 clean mostlyclean:
 	rm -f a.out core *.i *.o *-test *.srec *.dis *.x $(SIM_BSP) $(IQ80310_BSP)

libgloss/multi-build.in

@@ -1,8 +1,13 @@
+# This is like ../multilib.am and ../config-ml.in and ../config/multi.m4 mashed
+# into one file.  It relies on some custom hardcoded MULTI* variables in the
+# respective Makefile.in instead of dynamic compiler probing that ../multilib.am
+# does.
+
 PWD_COMMAND=$${PWDCMD-pwd}
 
 # FIXME: There should be an @-sign in front of the `if'.
 # Leave out until this is tested a bit more.
-multi-do:
+libgloss-multi-do:
 	if [ -z "$(MULTIDIRS)" ]; then \
 	  true; \
 	else \