simd/archsimd: define ToMask only on integer vectors

simd/archsimd: define ToMask only on integer vectors

The ToMask method is for converting an AVX2-style mask
represented in a vector to the Mask type. The AVX2-style mask is
a (signed) integer, so define ToMask only on integer vectors.

Cherry-pick CL 729020 from the dev.simd branch, for Go 1.26.

Change-Id: I0c541eb28e945bfaebf2a2feb940bdd438fb6e99
Reviewed-on: https://go-review.googlesource.com/c/go/+/729222
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: David Chase <drchase@google.com>