v0.8.79

fix(win): bundle Boehm GC into gcc-bundle so amc links out of the box

The winlibs MinGW toolchain shipped in the MSI has no Boehm GC, so a fresh
amc could compile a program to C but not LINK it on Windows (gc.h not found,
-lgc unresolved). Graft MSYS2 UCRT64's static libgc.a + headers into the
bundle's own include/lib (ABI-compatible with the winlibs ucrt GCC), where
the bundled gcc finds them automatically.

- release.yml: new step grafts gc.h + gc/ + libgc.a (+ libatomic_ops) into
  the gcc-bundle, then a smoke-test compiles & runs a GC program with the
  bundled gcc to prove the static link works before shipping the MSI.
- _runtime.h: define GC_NOT_DLL on Windows so gc.h targets the static lib
  (self-sufficient .exe, no gc.dll alongside).
- Bump AmcVersion 0.8.78 → 0.8.79.

Unblocks the FF-TAROT Windows build (server.c → gcc link -lgc).