avr32: Makefile: add '-D__linux__' flag for gcc-4.4.7 use

For avr32 cross compiler, do not define '__linux__' internally, so it
will cause issue with allmodconfig.

The related error:

    CC [M]  fs/coda/psdev.o
  In file included from include/linux/coda.h:64,
                   from fs/coda/psdev.c:45:
  include/uapi/linux/coda.h:221: error: expected specifier-qualifier-list before 'u_quad_t'

The related toolchain version (which only download, not re-compile):

  [root@gchen linux-next]# /upstream/toolchain/download/avr32-gnu-toolchain-linux_x86/bin/avr32-gcc -v
  Using built-in specs.
  Target: avr32
  Configured with: /data2/home/toolsbuild/jenkins-knuth/workspace/avr32-gnu-toolchain/src/gcc/configure --target=avr32 --host=i686-pc-linux-gnu --build=x86_64-pc-linux-gnu --prefix=/home/toolsbuild/jenkins-knuth/workspace/avr32-gnu-toolchain/avr32-gnu-toolchain-linux_x86 --enable-languages=c,c++ --disable-nls --disable-libssp --disable-libstdcxx-pch --with-dwarf2 --enable-version-specific-runtime-libs --d...