m32r: use __stringify() macro in assembler.h

Replace #x with __stringify(x).
Also, #ifndef __STR is removed and undefine __STR macro at the beginning.

The __STR() macro is still remained, because the assembler.h might be
included from assembly codes as well as C codes.

Signed-off-by: Hirokazu Takata <takata@linux-m32r.org>