v0.8.81

fix(win): amc build links out of the box — quoting + GC amalgamation

Two Windows blockers for `amc build` (and `amc test`):

1. Command paths were single-quoted. cmd.exe doesn't strip single quotes,
   so 'C:/Program Files/Amalgame/bin/amc.exe' split on the space →
   'C:/Program' n'est pas reconnu. main.am now double-quotes every gcc/amc
   sub-command path (cmd.exe + sh both honour those). -ldl (no mingw libdl)
   is dropped on Windows via a new Program.IsWindows() guard.

2. GC link: the 0.8.80 CMake build of bdwgc failed opaquely. Replace it with
   the deterministic single-TU amalgamation (extra/gc.c + gcc + ar), built
   with GC_WIN32_PTHREADS so the static libgc.a exports GC_pthread_create/
   _join (amalgame-threading needs them). The smoke test now force-resolves
   those symbols too — a non-threaded GC would fail the link.

Verified: amc build hello.am green on Linux (double quotes + -ldl kept).
Bump 0.8.80 → 0.8.81.