mk: Fix building compiler-rt on MSVC

It looks like compiler-rt has a cmake build sytem inside its source, but I have
been unable to figure out how to use it and actually build the right library.
For now this commit hard-wires MSVC-targeting builds of libcompiler-rt to
continue using `make` as the primary bulid system, but some frobbing of the
flags are necessary to ensure that the right compiler is used.

Author: alexcrichton

mk/rt.mk

@@ -226,18 +226,32 @@ COMPRT_NAME_$(1) := $$(call CFG_STATIC_LIB_NAME_$(1),compiler-rt)
 COMPRT_LIB_$(1) := $$(RT_OUTPUT_DIR_$(1))/$$(COMPRT_NAME_$(1))
 COMPRT_BUILD_DIR_$(1) := $$(RT_OUTPUT_DIR_$(1))/compiler-rt
 
+# Note that on MSVC-targeting builds we hardwire CC/AR to gcc/ar even though
+# we're targeting MSVC. This is because although compiler-rt has a CMake build
+# config I can't actually figure out how to use it, so I'm not sure how to use
+# cl.exe to build the objects. Additionally, the compiler-rt library when built
+# with gcc has the same ABI as cl.exe, so they're largely compatible
+COMPRT_CC_$(1) := $$(CC_$(1))
+COMPRT_AR_$(1) := $$(AR_$(1))
+COMPRT_CFLAGS_$(1) := $$(CFG_GCCISH_CFLAGS_$(1))
+ifeq ($$(findstring msvc,$(1)),msvc)
+COMPRT_CC_$(1) := gcc
+COMPRT_AR_$(1) := ar
+COMPRT_CFLAGS_$(1) := $$(CFG_GCCISH_CFLAGS_$(1)) -m64
+endif
+
 $$(COMPRT_LIB_$(1)): $$(COMPRT_DEPS) $$(MKFILE_DEPS)
 	@$$(call E, make: compiler-rt)
 	$$(Q)$$(MAKE) -C "$(S)src/compiler-rt" \
 		ProjSrcRoot="$(S)src/compiler-rt" \
 		ProjObjRoot="$$(abspath $$(COMPRT_BUILD_DIR_$(1)))" \
-		CC="$$(CC_$(1))" \
-		AR="$$(AR_$(1))" \
-		RANLIB="$$(AR_$(1)) s" \
-		CFLAGS="$$(CFG_GCCISH_CFLAGS_$(1))" \
+		CC='$$(COMPRT_CC_$(1))' \
+		AR='$$(COMPRT_AR_$(1))' \
+		RANLIB='$$(COMPRT_AR_$(1)) s' \
+		CFLAGS="$$(COMPRT_CFLAGS_$(1))" \
 		TargetTriple=$(1) \
 		triple-builtins
-	$$(Q)cp $$(COMPRT_BUILD_DIR_$(1))/triple/builtins/libcompiler_rt.a $$(COMPRT_LIB_$(1))
+	$$(Q)cp $$(COMPRT_BUILD_DIR_$(1))/triple/builtins/libcompiler_rt.a $$@
 
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 # libbacktrace