Fix MIPS cross-compile problem

Crosscompiling on a Fedora 9 machine running gcc 4.3.0 as its host compiler
and gcc 3.4.6 for the mips-linux target results in the following build
error:

$ make malta_defconfig
$ make
cc1: error: unrecognized command line option "-fno-stack-protector"
scripts/kconfig/conf -s arch/mips/Kconfig
cc1: error: unrecognized command line option "-fno-stack-protector"

The arch Makefile is included too late so the host compiler is feature
tested, not the crosscompiler as intended and thus the Makefile applies
adds -fno-stack-protector to crosscompiler's flags which fails for gcc
3.4.6.  The bug was introduced by e06b8b98
in 2.6.25; 35bb5b1e did add more flags
testing before the arch Makefile inclusion.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>