[MIPS] There is no __GNUC_MAJOR__

Gcc major version number is in __GNUC__. As side effect fix checking
with sparse if sparse was built with gcc 4.1 and mips cross-compiler
is 3.4.

Sparse will inherit version 4.1, __GNUC__ won't be filtered from
"-dM -E -xc" output, sparse will pick only new major, effectively becoming
gcc version 3.1 which is unsupported.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>