tools build: Add support for host programs format

In some cases, like for fixdep and shortly for jevents, we need to build a tool
to run on the host that will be used in building a tool, such as perf, that is
being cross compiled, so do like the kernel and provide HOSTCC, HOSTLD and HOSTAR
to do that.

Signed-off-by: Jiri Olsa <[e-mail]>
Requested-by: Andi Kleen <[e-mail]>
Requested-and-Tested-by: Arnaldo Carvalho de Melo <[e-mail]>
Cc: Madhavan Srinivasan <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Sukadev Bhattiprolu <[e-mail]>
Cc: [e-mail]
Link: http://lkml.kernel.org/r/20160927141846.GA6589@krava
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/build/Build.include

···
 # - per object C flags
 # - BUILD_STR macro to allow '-D"$(variable)"' constructs
 c_flags = -Wp,-MD,$(depfile),-MT,$@ $(CFLAGS) -D"BUILD_STR(s)=\#s" $(CFLAGS_$(basetarget).o) $(CFLAGS_$(obj))
+
+###
+## HOSTCC C flags
+
+host_c_flags = -Wp,-MD,$(depfile),-MT,$@ $(CHOSTFLAGS) -D"BUILD_STR(s)=\#s" $(CHOSTFLAGS_$(basetarget).o) $(CHOSTFLAGS_$(obj))

tools/build/Makefile

···
 $(call allow-override,CC,$(CROSS_COMPILE)gcc)
 $(call allow-override,LD,$(CROSS_COMPILE)ld)
 
+HOSTCC ?= gcc
+HOSTLD ?= ld
+HOSTAR ?= ar
+
+export HOSTCC HOSTLD HOSTAR
+
 ifeq ($(V),1)
   Q =
 else

tools/build/Makefile.build

···
 quiet_cmd_cc_o_c = CC       $@
       cmd_cc_o_c = $(CC) $(c_flags) -c -o $@ $<
 
+quiet_cmd_host_cc_o_c = HOSTCC   $@
+      cmd_host_cc_o_c = $(HOSTCC) $(host_c_flags) -c -o $@ $<
+
 quiet_cmd_cpp_i_c = CPP      $@
       cmd_cpp_i_c = $(CC) $(c_flags) -E -o $@ $<
 
···
 # If there's nothing to link, create empty $@ object.
 quiet_cmd_ld_multi = LD       $@
       cmd_ld_multi = $(if $(strip $(obj-y)),\
-		       $(LD) -r -o $@  $(filter $(obj-y),$^),rm -f $@; $(AR) rcs $@)
+                     $(LD) -r -o $@  $(filter $(obj-y),$^),rm -f $@; $(AR) rcs $@)
+
+quiet_cmd_host_ld_multi = HOSTLD   $@
+      cmd_host_ld_multi = $(if $(strip $(obj-y)),\
+                          $(HOSTLD) -r -o $@  $(filter $(obj-y),$^),rm -f $@; $(HOSTAR) rcs $@)
+
+ifneq ($(filter $(obj),$(hostprogs)),)
+  host = host_
+endif
 
 # Build rules
 $(OUTPUT)%.o: %.c FORCE
 	$(call rule_mkdir)
-	$(call if_changed_dep,cc_o_c)
+	$(call if_changed_dep,$(host)cc_o_c)
 
 $(OUTPUT)%.o: %.S FORCE
 	$(call rule_mkdir)
-	$(call if_changed_dep,cc_o_c)
+	$(call if_changed_dep,$(host)cc_o_c)
 
 $(OUTPUT)%.i: %.c FORCE
 	$(call rule_mkdir)
···
 
 $(in-target): $(obj-y) FORCE
 	$(call rule_mkdir)
-	$(call if_changed,ld_multi)
+	$(call if_changed,$(host)ld_multi)
 
 __build: $(in-target)
 	@: