Windows: GCCToolchain_jll v15.2.0+2 (unaligned vector moves)

[maleadt]

Moves both Windows images to GCCToolchain_jll v15.2.0+2 (JuliaPackaging/Yggdrasil#14921). Its Windows toolchains carry MSYS2's -mno-align-vector-insn patch, enabled by default, as in the MSYS2 GCC that v8.5 used.

Without it, code built with AVX enabled can crash on 64-bit Windows. GCC uses aligned vector moves for 32-byte AVX stack slots, which it cannot align there (GCC PR 54412). Julia's Windows coverage job builds with -march=native and hit this in v8.9: julia.exe crashed at vmovdqa %ymm0,0xb0(%rsp) on its first run (JuliaCI/julia-buildkite#636, julia-buildkite-ci build 182). The regular builds use a baseline -march and were unaffected.

Validated locally in images that differ from v8.9 only by this toolchain:

• The coverage-style build of that Julia commit now succeeds.
• The CI-style x86_64 and i686 builds succeed, and pass the core, numbers, strings, math, misc, threads, ccall and exceptions tests.