Makefile: properly set the ARCH variable when forcing a DESTCPU

Makefile

@@ -193,7 +193,15 @@ docclean:
 
 VERSION=v$(shell $(PYTHON) tools/getnodeversion.py)
 PLATFORM=$(shell uname | tr '[:upper:]' '[:lower:]')
+ifeq ($(DESTCPU),x64)
+ARCH=x86_64
+else
+ifeq ($(DESTCPU),ia32)
+ARCH=i386
+else
 ARCH=$(shell uname -m)
+endif
+endif
 TARNAME=node-$(VERSION)
 TARBALL=$(TARNAME).tar.gz
 BINARYNAME=$(TARNAME)-$(PLATFORM)-$(ARCH)