m68knommu: rename BSS define in linker script

The "BSS" define name now used in asm-generic/vmlinux.lds.h
(introduced in commit ef53dae8)
clashes with the internal "BSS" define in the m68knommu vmlinux.lds.S
linker script. So rename it.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>