[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>