ci: use cross's edge images for multiarch test runs

[ghedo]

The default cross images shipped with cross 0.2.5 (2022) carry pre-C++17 g++ toolchains, which can't compile the upgraded BoringSSL (it needs C++17 headers like <string_view>). Override them with cross's edge images, which are rebuilt regularly from cross's main branch and carry a current toolchain.

The edge image for i686-unknown-linux-gnu is set up differently from the others: it's a 32-bit-native userspace shipping only the unprefixed gcc/g++ toolchain, missing some of the 32-bit dev libraries needed by cmake's compiler-detection probe, and lacks the cross-prefixed binary names that cross and cc-rs expect. Add a pre-build step to install libc6-dev-i386 g++-multilib and to symlink i686-linux-gnu-gcc/-g++ to the unprefixed compilers.

Separately, BoringSSL's x86 assembly requires SSE2. The 32-bit toolchain file sets -msse2, but cmake-rs passes
-DCMAKE_C_FLAGS=... on the command line which wins over the toolchain file's CACHE STRING, leaving SSE2 unset in the actual build flags. Add -msse2 (and -mfpmath=sse) for the x86 build path in build.rs so they make it into the final flags regardless.

An earlier attempt switched the multiarch CI job from cross to native apt cross-toolchains plus qemu-user-static. That approach worked but qemu-user-static introduced enough nondeterminism in microsecond-level timing that several BBR2-parameterized byte-count assertions started flaking on aarch64 and armv7 across different runs of the same code. cross's Docker+QEMU setup is deterministic enough in practice to keep those tests stable, so we keep using cross and address its toolchain age via Cross.toml.

Split-off from #2446.