sh64: Drop deprecated ISA tuning for legacy toolchains.

The original sh64 toolchains required that we tune the ISA
level accordingly to not have head.S/entry.S blow up. With
current toolchains, this is no longer the case, and the
syntax magically changed as well, causing all current
toolchains to die a horrible death.

Incidentally, code generation in other parts of the kernel
is now significantly complex enough that none of the older
toolchains make it very far these days, so there's not
even any point in preserving legacy compatability via
as-option.

This fixes a long-standing issue, as noted here:

	http://lkml.org/lkml/2005/1/5/223



Though at the time the current toolchains were too broken
to make adjusting the tuning worthwhile.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>