Ports: Add a bdwgc (aka libgc) port

For all your ports' garbage collection needs :^)

Ports/bdwgc/patches/ReadMe.md

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+# Patches for bdwgc on SerenityOS
+
+## `0001-Teach-os_dep-and-gcconfig.h-about-serenity.patch`
+
+Teach os_dep and gcconfig.h about serenity
+
+
+## `0002-Error-on-unknown-arch.patch`
+
+Error on unknown arch
+
+
+## `0003-Teach-dyn_load.c-about-serenity.patch`
+
+Teach dyn_load.c about serenity
+
+
+## `0004-Teach-bdwgc-about-serenity-signals.patch`
+
+Teach bdwgc about serenity signals
+
+Serenity doesn't have the realtime POSIX signals, so use SIGXCPU and
+SIGXFSZ instead.
+
+## `0005-Explicitly-link-with-pthread.patch`
+
+Explicitly link with pthread
+
+The cmakelists was using the wrong variable to link against pthread.
+
+## `0006-Make-the-collector-build-with-threads.patch`
+
+Make the collector build with threads
+
+In an extremely limited way for now:
+- No extra threads
+    More threads always lead to exactly one borked thread that's stuck
+    in no man's land, doing who-knows-what, and definitely not
+    responding to signals.
+    However, the APIs are there and they work, so they *can* be used to
+    make threads.
+- No fork handling
+    Seems borked for unknown reasons.
+