sparc32: Add -Wa,-Av8 to KBUILD_CFLAGS.

Binutils used to be (erroneously) extremely permissive about
instruction usage.  But that got fixed and if you don't properly tell
it to accept classes of instructions it will fail.

This uncovered a specs bug on sparc in gcc where it wouldn't pass the
proper options to binutils options.

Deal with this in the kernel build by adding -Wa,-Av8 to KBUILD_CFLAGS.

Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>