SSE2 required for i686 targets in rust >= 1.86

[Jan Palus]

Starting with rust 1.86 binaries built for i686 (including rustc itself)
require SSE2 instructions (refer to release notes for rationale). The
question is how we'd like to handle this:

1. Change target to i586-unknown-linux-gnu for i686.
   i586-unknown-linux-gnu is "Tier 2 platform without host tools" so
   we'd need to build both i686-unknown-linux-gnu for tools which would
   require SSE2 as well as i586-unknown-linux-gnu.

2. Put "Requires: cpuinfo(sse2)" possibly through some %rust_req macro
   in every package utilizing rust

3. Change our baseline for i686 so SSE2 is mandatory and do nothing.

Any other ideas are welcome.