Oops. We can't do this yet until the next snapshot.

Closes #2042
Closes #1981

intrinsics.cpp and the file to generate the ll had bitrotted.

This intrinsic existed just to get ahold of the return pointer. I replaced it
with a call_with_retptr intrinsic that grabs the return pointer and passes it
to another Rust function, thereby eliminating the need to call C functions on
the Rust stack.

there is one test failure, stdtest/sys.rs, which inexplicably
(thus far) fails to compile because it invokes
sys::rustrt::last_os_error() instead of invoking
sys::last_os_error().  If stdtest/sys.rs is updated to invoke
the wrapper, it passes.  Still tracing the source of this error.

Only intrinsics still take a dummy taskptr. We'll have to do some
makefile stunts to snapshot a version without taskptrs-in-intrinsics.

Issue #466

rt: Add an upcall to allocate space on the C stack. This will be used for native calls on the C stack.

Vectors are now similar to our old, pre-internal vectors, except that
they are uniquely owned, not refcounted.

Their name should probably change too, then. I've renamed them to vec
in the runtime, will do so throughout the compiler later.

I just edited the ll by hand since I don't have clang on hand

rustc: Stub Rust intrinsics. We check in the .ll file for now to temporarily dodge a dependency on clang.