m32r: Define THREAD_SIZE only once.

Previously, m32r's asm/thread_info.h defined THREAD_SIZE differently
for assembly and C code; now that PAGE_SIZE is usable from assembly,
these can be combined.  Also, m32r's asm/processor.h redefines
THREAD_SIZE to the same value; remove this redundant definition.

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