mk/rt: use CFG_LLVM_TARGET instead of plain target when calling llc

We add CFG_LLVM_TARGET_$(target) (which can be defined in any of the
mk/cfg/* files) and supply a default to the plain target name

CFG_LLVM_TARGET mirrors the value of llvm_target (aka llvm-target) in
the librustc_back runtime target specification.

mk/main.mk

@@ -190,11 +190,14 @@ endif
 # Target-and-rule "utility variables"
 ######################################################################
 
-define DEF_X
+define DEF_FOR_TARGET
 X_$(1) := $(CFG_EXE_SUFFIX_$(1))
+ifndef CFG_LLVM_TARGET_$(1)
+CFG_LLVM_TARGET_$(1) := $(1)
+endif
 endef
 $(foreach target,$(CFG_TARGET), \
-  $(eval $(call DEF_X,$(target))))
+  $(eval $(call DEF_FOR_TARGET,$(target))))
 
 # "Source" files we generate in builddir along the way.
 GENERATED :=

mk/rt.mk

@@ -75,7 +75,7 @@ $$(RT_OUTPUT_DIR_$(1))/%.o: $(S)src/rt/%.ll $$(MKFILE_DEPS) \
 	@mkdir -p $$(@D)
 	@$$(call E, compile: $$@)
 	$$(Q)$$(LLC_$$(CFG_BUILD)) $$(CFG_LLC_FLAGS_$(1)) \
-	    -filetype=obj -mtriple=$(1) -relocation-model=pic -o $$@ $$<
+	    -filetype=obj -mtriple=$$(CFG_LLVM_TARGET_$(1)) -relocation-model=pic -o $$@ $$<
 
 $$(RT_OUTPUT_DIR_$(1))/%.o: $(S)src/rt/%.c $$(MKFILE_DEPS)
 	@mkdir -p $$(@D)