lib9, libmach: Change GOOS references to GOHOSTOS

This makes it possible to build Windows binaries on non-Windows OSes.

R=golang-dev, rsc1, rsc
CC=golang-dev
https://golang.org/cl/4271046

src/lib9/Makefile

@@ -82,7 +82,7 @@ LIB9OFILES=\
 	time.$O\
 	tokenize.$O\
 
-ifeq ($(GOOS),windows)
+ifeq ($(GOHOSTOS),windows)
 LIB9OFILES+=\
 	win32.$O\
 

src/libmach/Makefile

@@ -49,7 +49,7 @@ OFILES=\
 	6obj.$O\
 	8obj.$O\
 
-ifneq ($(GOOS),windows)
+ifneq ($(GOHOSTOS),windows)
 OFILES+=\
 	$(shell uname | tr A-Z a-z).$O\