Automate building QEMU

We need QEMU to run the test suite.  This patch automates the build so
I don't need to keep remembering how to do it by hand.

Author: palmer-dabbelt

.gitmodules

@@ -13,3 +13,6 @@
 [submodule "riscv-newlib"]
 	path = riscv-newlib
 	url = ../riscv-newlib.git
+[submodule "riscv-qemu"]
+	path = riscv-qemu
+	url = git://github.com/riscv/riscv-qemu.git

Makefile.in

@@ -290,15 +290,24 @@ stamps/build-gcc-newlib-stage2: $(srcdir)/riscv-gcc stamps/build-newlib
 	$(MAKE) -C $(notdir $@) install
 	mkdir -p $(dir $@) && touch $@
 
-.PHONY: check-gcc-newlib
-check-gcc-newlib: stamps/build-gcc-newlib-stage2
+stamps/build-qemu: $(srcdir)/riscv-qemu
+	rm -rf $@ $(notdir $@)
+	mkdir $(notdir $@)
+	cd $(notdir $@) && $</configure \
+		--prefix=$(INSTALL_DIR) \
+		--target-list=riscv64-linux-user,riscv32-linux-user
+	$(MAKE) -C $(notdir $@)
+	$(MAKE) -C $(notdir $@) install
+	date > $@
+
+stamps/check-gcc-newlib: stamps/build-gcc-newlib-stage2 stamps/build-qemu
 	export PATH=$(srcdir)/scripts/wrapper/qemu:$(INSTALL_DIR)/bin:$(PATH) && \
 	export DEJAGNULIBS=$(srcdir)/riscv-dejagnu && \
 	export RISC_V_SYSROOT=$(SYSROOT) && \
 	cd build-gcc-newlib-stage2 && \
 	$(MAKE) check-gcc "RUNTESTFLAGS=--target_board=riscv-sim"
 
-check-gcc-linux: stamps/build-gcc-linux-stage2
+stamps/check-gcc-linux: stamps/build-gcc-linux-stage2 stamps/build-qemu
 	export PATH=$(srcdir)/scripts/wrapper/qemu:$(INSTALL_DIR)/bin:$(PATH) && \
 	export DEJAGNULIBS=$(srcdir)/riscv-dejagnu && \
 	export RISC_V_SYSROOT=$(SYSROOT) && \