m32r: Move the spi_stack_top and spu_stack_top into .init.data section.

Since these get squashed into the .data output section by the m32r
linker script, it seems likely that they don't need their own input
sections.

At Hirokazu Takata's suggestion, we place these structures in
.init.data rather than just placing them in .data (since they are only
used at init time).

This patch is preparation for cleaning up the m32r architecture to use
the new macros in vmlinux.lds.h; if these sections are indeed not
needed, then we can use the RW_DATA_SECTION macro for m32r and save a
bunch of redundant code.

Signed-off-by: Tim Abbott <tabbott@ksplice.com>
Signed-off-by: Hirokazu Takata <takata@linux-m32r.org>