Exclude alignment for MaskBuffer for i686-win7-windows-msvc

[drewkett]

Windows 7 does not respect 16 byte alignment for thread locals on i686
builds. Rust 1.79 changed i686 windows builds to use native thread local
support. As a result, using matrixmultiply on i686 win7 builds leads
to a UB check panic nounwind when run on Windows 7. See
rust-lang/rust#138903 for more info.

This change adds i686-win7-windows-msvc as an excluded target for the
alignment attribute on MaskBuffer.

Not sure how you feel about adding this exclusion in matrixmultiply as its
very much for a tier 3 target that you can't test in CI that its useful. But
figured I'd raise it, as it would affect users using that target on any recent
toolchain version. And fwiw, i did test that this does in fact have the desire
for compilations targeting i686-win7

[bluss]

Thanks, at this point we should have more or less abandoned any hope for that custom alignment. I will include your change.