Unimplemented intrinsics required to codegen Firecracker vmm

[nchong-at-aws]

Not implemented intrinsics:

• atomic_xchg
• atomic_xchg_acq
• atomic_xchg_acqrel
• atomic_xchg_rel
• atomic_xchg_relaxed
• try

[adpaco-aws]

We have atomic_cxchg* implemented so it should be easy to support these atomic intrinsics. try seems a little harder though.