powerpc: fix uname -m

It will now give ppc64 on 64bit platforms and ppc on 32bit
platforms.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
diff --git a/arch/powerpc/Makefile b/arch/powerpc/Makefile
index 8d1e7bd..a5f2eb5 100644
--- a/arch/powerpc/Makefile
+++ b/arch/powerpc/Makefile
@@ -51,6 +51,8 @@
 SZ	:= 32
 endif
 
+UTS_MACHINE := $(OLDARCH)
+
 ifeq ($(HAS_BIARCH),y)
 override AS	+= -a$(SZ)
 override LD	+= -m elf$(SZ)ppc