Makefiles: Don't define _64BIT now that 6c does it by default.

R=rsc
CC=golang-dev
https://golang.org/cl/3207041

src/Make.pkg

@@ -132,8 +132,7 @@ _cgo_export.o: _cgo_export.c _cgo_export.h
 # added _cgo_defun.$O to $OFILES, and added the installed copy of
 # package_x.so (built from x.cgo2.c) to $(INSTALLFILES).
 
-RUNTIME_CFLAGS_amd64=-D_64BIT
-RUNTIME_CFLAGS=-I"$(GOROOT)/src/pkg/runtime" $(RUNTIME_CFLAGS_$(GOARCH))
+RUNTIME_CFLAGS=-I"$(GOROOT)/src/pkg/runtime"
 
 # Have to run gcc with the right size argument on hybrid 32/64 machines.
 _CGO_CFLAGS_386=-m32

src/pkg/runtime/Makefile

@@ -12,13 +12,11 @@ SIZE_amd64=64
 SIZE_arm=32
 SIZE=$(SIZE_$(GOARCH))
 
-# Setup CFLAGS.  Add -D_64BIT on 64-bit platforms (sorry).
-CFLAGS_64=-D_64BIT
 # TODO(kaib): fix register allocation to honor extern register so we
 # can enable optimizations again.
 CFLAGS_arm=-N
 CFLAGS_windows=-D__WINDOWS__
-CFLAGS=-I$(GOOS) -I$(GOARCH) -I$(GOOS)/$(GOARCH) -wF $(CFLAGS_$(SIZE)) $(CFLAGS_$(GOARCH)) $(CFLAGS_$(GOOS))
+CFLAGS=-I$(GOOS) -I$(GOARCH) -I$(GOOS)/$(GOARCH) -wF $(CFLAGS_$(GOARCH)) $(CFLAGS_$(GOOS))
 
 GOFILES=\
 	debug.go\