m32r: Move GET_THREAD_INFO definition out of asm/thread_info.h.

Previously, asm/thread_info.h was not usable from linker scripts
because it contains a piece of .macro code.  Since that code was only
used in the m32r entry.S, the right fix is probably to move the macro
there.

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