libmach: cross compiling support

        We already use GOHOSTOS to represent the host OS that the toolchain
        will be run on, so no need to resort to uname(1) to get that (and
        use uname(1) will make cross-compiling for another host impossible).

R=rsc, golang-dev
CC=golang-dev
https://golang.org/cl/5530050

src/libmach/Makefile

@@ -49,13 +49,13 @@ OFILES=\
 	6obj.$O\
 	8obj.$O\
 
-ifneq ($(GOHOSTOS),windows)
+ifneq ($(GOHOSTOS),)
 OFILES+=\
-	$(shell uname | tr A-Z a-z).$O\
+	$(GOHOSTOS).$O\
 
 else
 OFILES+=\
-	windows.$O\
+	$(shell uname | tr A-Z a-z).$O\
 
 endif