support clean/distclean make targets in unconfigured tree

commit 2f853dd6b9 moved the error
handling for $(ARCH) not being set such that it applied to all
targets, including clean and distclean. previously these targets
worked even in an unconfigured tree. to restore the old behavior, make
most of the makefile body conditional on $(ARCH) being set/non-empty
and produce the error via a fake "all" target in the conditional
branch for the case where $(ARCH) is empty.

Makefile

@@ -77,8 +77,12 @@ LDSO_PATHNAME = $(syslibdir)/ld-musl-$(ARCH)$(SUBARCH).so.1
 -include config.mak
 
 ifeq ($(ARCH),)
-$(error Please set ARCH in config.mak before running make.)
-endif
+
+all:
+	@echo "Please set ARCH in config.mak before running make."
+	@exit 1
+
+else
 
 all: $(ALL_LIBS) $(ALL_TOOLS)
 
@@ -89,14 +93,6 @@ $(ALL_LIBS) $(ALL_TOOLS) $(ALL_OBJS) $(ALL_OBJS:%.o=%.lo) $(GENH) $(GENH_INT): |
 $(OBJ_DIRS):
 	mkdir -p $@
 
-install: install-libs install-headers install-tools
-
-clean:
-	rm -rf obj lib
-
-distclean: clean
-	rm -f config.mak
-
 obj/include/bits/alltypes.h: $(srcdir)/arch/$(ARCH)/bits/alltypes.h.in $(srcdir)/include/alltypes.h.in $(srcdir)/tools/mkalltypes.sed
 	sed -f $(srcdir)/tools/mkalltypes.sed $(srcdir)/arch/$(ARCH)/bits/alltypes.h.in $(srcdir)/include/alltypes.h.in > $@
 
@@ -223,10 +219,20 @@ install-headers: $(ALL_INCLUDES:include/%=$(DESTDIR)$(includedir)/%)
 
 install-tools: $(ALL_TOOLS:obj/%=$(DESTDIR)$(bindir)/%)
 
+install: install-libs install-headers install-tools
+
 musl-git-%.tar.gz: .git
 	 git --git-dir=$(srcdir)/.git archive --format=tar.gz --prefix=$(patsubst %.tar.gz,%,$@)/ -o $@ $(patsubst musl-git-%.tar.gz,%,$@)
 
 musl-%.tar.gz: .git
 	 git --git-dir=$(srcdir)/.git archive --format=tar.gz --prefix=$(patsubst %.tar.gz,%,$@)/ -o $@ v$(patsubst musl-%.tar.gz,%,$@)
 
+endif
+
+clean:
+	rm -rf obj lib
+
+distclean: clean
+	rm -f config.mak
+
 .PHONY: all clean install install-libs install-headers install-tools