um: Run host archheaders, allow use of host generated headers

Run the "archheaders" target for the host architecture, for
architectures (like x86, now) that want to generate some of the
necessary header files.

Add $(HOST_DIR)/include/generated to the include path so we then pick
them up.
Signed-off-by: NH. Peter Anvin <hpa@linux.intel.com>

arch/um/Makefile

@@ -64,7 +64,8 @@ USER_CFLAGS = $(patsubst $(KERNEL_DEFINES),,$(patsubst -D__KERNEL__,,\
 #This will adjust *FLAGS accordingly to the platform.
 include $(srctree)/$(ARCH_DIR)/Makefile-os-$(OS)
 
-KBUILD_CPPFLAGS += -I$(srctree)/$(HOST_DIR)/include
+KBUILD_CPPFLAGS += -I$(srctree)/$(HOST_DIR)/include \
+		   -I$(HOST_DIR)/include/generated
 
 # -Derrno=kernel_errno - This turns all kernel references to errno into
 # kernel_errno to separate them from the libc errno.  This allows -fno-common
@@ -96,6 +97,10 @@ endef
 
 KBUILD_KCONFIG := $(HOST_DIR)/um/Kconfig
 
+archheaders:
+	$(Q)$(MAKE) -C '$(KBUILD_SRC)' KBUILD_SRC= \
+		ARCH=$(SUBARCH) O='$(objtree)' archheaders
+
 archprepare: include/generated/user_constants.h
 
 LINK-$(CONFIG_LD_SCRIPT_STATIC) += -static