x86-64, build: Fix stack protector Makefile breakage with 32-bit userland

If you are using a 64-bit kernel with 32-bit userland, then
scripts/gcc-x86_64-has-stack-protector.sh invokes 32-bit gcc
with -mcmodel=kernel, which produces:

<stdin>:1:0: error: code model 'kernel' not supported in the 32 bit mode

and trips the "broken compiler" test at arch/x86/Makefile:120.

There are several places a fix is possible, but the following seems
cleanest. (But it's minimal; it would also be possible to factor
out a bunch of stuff from the two branches of the if.)

Signed-off-by: George Spelvin <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Cc: <[e-mail]> # v3.14
Signed-off-by: H. Peter Anvin <[e-mail]>

arch/x86/Makefile

···
         UTS_MACHINE := x86_64
         CHECKFLAGS += -D__x86_64__ -m64
 
+        biarch := -m64
         KBUILD_AFLAGS += -m64
         KBUILD_CFLAGS += -m64