v0.8.80

fix(win): build Boehm GC from source with the bundled gcc (ABI match)

The prebuilt MSYS2 libgc.a failed to link against the winlibs toolchain
(undefined __imp__setjmp — different mingw-w64 CRT). The 0.8.79 smoke test
caught it before shipping. Fix: build bdwgc 8.2.8 from source in CI with the
*bundled winlibs gcc itself*, as a static lib with GCC builtin atomics
(no libatomic_ops), then graft libgc.a + headers into the gcc-bundle. The
smoke test now passes the static link because the lib and the toolchain share
one CRT. Bump 0.8.79 → 0.8.80.