[POWERPC] Fix building of COFF zImages

The COFF zImage (for booting oldworld powermacs) wasn't being built
correctly because the procedure descriptor in crt0.S for the zImage
entry point wasn't declared as .globl, and therefore wasn't getting
pulled in from wrapper.a by the linker.  This adds the necessary
.globl statement.
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/boot/crt0.S

@@ -13,6 +13,7 @@
 
 	.text
 	/* a procedure descriptor used when booting this as a COFF file */
+	.globl	_zimage_start_opd
 _zimage_start_opd:
 	.long	_zimage_start, 0, 0, 0