Teach os_dep and gcconfig.h about serenity

Error on unknown arch

Teach dyn_load.c about serenity

Teach bdwgc about serenity signals

Serenity doesn't have the realtime POSIX signals, so use SIGXCPU and SIGXFSZ instead.

Explicitly link with pthread

The cmakelists was using the wrong variable to link against pthread.

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.

Add serenity to the conigure list of pthread unixes