[PATCH] add initial support for J2 core to sh target

[Rich Felker <dalias at libc dot org>]

The J2 Core is an open hardware cpu implementing the SH-2 instruction
set, with the addition of barrel shift instructions and an atomic
compare-and-swap instruction. This patch adds a cpu model option -mj2
to the sh target. Presently all it does is enable use of the barrel
shift instructions (and turns off assembler checking of the ISA via
--isa=any) but I will eventually add support for the new CAS
instruction as a new -matomic-model for use by the __sync and __atomic
builtins.

I've used the the name "J2" and "-mj2" rather than treating it as a
submodel variant of "sh2" ("SH2J" and "-m2j") because the official
name of this cpu model is "J2", with the intent of not misrepresenting
it as a Renesas product. However I'd like feedback from GCC's side on
how GCC wants to identify J2 in cpu model options, tuples, and
internally; that part is not set in stone.

The --isa=any passed to the assembler probably needs to be fixed
before this patch is ready for upstream (although I'd really just
prefer _always_ passing --isa=any to the assembler, since the current
behavior breaks runtime-switching implementations, which I need). One
complication is that passing --isa=j2 will fail with old binutils; I'm
not sure if this should be detected and handled or if we should just
require up-to-date binutils to use -mj2. In any case, I don't yet have
an assembler-side patch to add the --isa level.

I know this isn't ready for upstream yet but I'd appreciate feedback
on the patch so far and anything that I need to change in order for it
to be acceptable.

Rich

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