openrisc: Makefile: append "-D__linux__" to KBUILD_CFLAGS

Need append "_D__linux__" to KBUILD_CFLAGS, just like some of another
architectures have done, or 'allmodconfig' can not pass compiling.

The related error:

    CC [M]  fs/coda/psdev.o
  In file included from include/linux/coda.h:65:0,
                   from fs/coda/psdev.c:45:
  include/uapi/linux/coda.h:221:2: error: expected specifier-qualifier-list before 'u_quad_t'

The related compiler information:

  [root@dhcp122 ~]# /usr/local/bin/or32-linux-gcc -v
  Using built-in specs.
  COLLECT_GCC=/usr/local/bin/or32-linux-gcc
  COLLECT_LTO_WRAPPER=/usr/local/libexec/gcc/or32-linux/4.5.1-or32-1.0rc1/lto-wrapper
  Target: or32-linux
  Configured with: ../gcc-openrisc/configure --target=or32-linux --disable-nls --enable-languages=c --without-headers --disable-shared --disable-threads --enable-werror=no
  Thread model: single
  gcc version 4.5.1-or32-1.0rc1 (GCC)

Signed-off-by: Chen Gang <gang.chen@asianux.com>
Signed-off-by: Jonas Bonn <jonas@southpole.se>