Need way to change kbuild CC and modify kbuild cflags.

We will use this to mark symbols as weak, so they can be redefined
easily.

Rather than do -fno-ipa-sra and -fno-ipa-cp (no interprocedural
analysis that removes unused arguments, constant propagation, etc.),
we can just turn it off on the function (thc_nested_async) that we
really care about. In this way also, libasync users don't need to
worry about setting these compiler flags.

These are no-ops for now, but we may want to have some global
initialization stuff later.

The only time we need to build libasync as a .ko is if we want
to share the same instance of the code amongst other .ko's, for
example. Not needed right now (can just statically link the whole
library).

Finished dispatch test code and got it building with autoconfig. Found minor bug in test. Needs to be fixed. I will probably revisit it again tomorrow.

I knew this would be a little painful, but time to get it
over with. Build not tested/fixed yet.

We can extend in the future as necessary. thc wasn't written for
our platform directory layout, and it's not worth ripping it apart
right now.

Also, reverted old changes back (uncommented asserts, etc.).

Started skeleton for dispatch loop tests using fipc. Thread3 setup and finishing thread2 setup is what's left for the test.

fixed stack overflow bug in ASYNC. This is a bug in Barrelfish. The RETURN_CONT macro was not popping the return address off of the stack when it should have. This would only happen when ASYNC was called a lot without anything yielding within it. I think this is why the bug was undiscovered.