[dev.simd] simd/_gen: migrate simdgen from x/arch

[dev.simd] simd/_gen: migrate simdgen from x/arch

This moves the simdgen tool and its supporting unify package from
golang.org/x/arch/internal as of CL 695619 to simd/_gen in the main repo.

The simdgen tool was started in x/arch to live next to xeddata and a
few other assembler generators that already lived there. However, as
we've been developing simdgen, we've discovered that there's a
tremendous amount of process friction coordinating commits to x/arch
with the corresponding generated files in the main repo.

Many of the existing generators in x/arch were started before modules
existed. In GOPATH world, it was impractical for them to live in the
main repo because they have dependencies that are not allowed in the
main repo. However, now that we have modules and can use small
submodules in the main repo, we can isolate these dependencies to just
the generators, making it practical for them to live in the main repo.

This commit was generated by the following script:

	# Checks
	set -e
	if [[ ! -d src/simd ]]; then
	    echo >&2 "$PWD is not the root of the main repo on dev.simd"
	    exit 1
	fi
	if [[ -z "$XEDDATA" ]]; then
	    echo >&2 "Must set \$XEDDATA"
	    exit 1
	fi
	which go >/dev/null

	# Move simdgen from x/arch
	xarch=$(mktemp -d)
	git clone https://go.googlesource.com/arch $xarch
	xarchCL=$(git -C $xarch log -1 --format=%b | awk -F/ '/^Reviewed-on:/ {print $NF}')
	echo >&2 "x/arch CL: $xarchCL"
	mv $xarch/internal src/simd/_gen
	sed --in-place s,golang.org/x/arch/internal/,simd/_gen/, src/simd/_gen/*/*.go
	# Create self-contained module
	cat > src/simd/_gen/go.mod <<EOF
	module simd/_gen

	go 1.24
	EOF
	cd src/simd/_gen
	go mod tidy
	git add .
	git gofmt
	# Regenerate file
	go run -C simdgen . -xedPath $XEDDATA -o godefs -goroot $(go env GOROOT) go.yaml types.yaml categories.yaml
	go run -C ../../cmd/compile/internal/ssa/_gen .

Change-Id: I56dd8473e913a9eb1978d9b3b3518ed632972f6f
Reviewed-on: https://go-review.googlesource.com/c/go/+/695975
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>