[RFC] rump hypcall integration for LKL

[thehajime]

(since this is RFC, I'd be happy to have any comments and suggestions from yours)

This PR provides an initial implementation of rump kernel hypercalls (*1) which brings us a couple of interesting features to LKL.

The highlight of new features are:

• additional host environments, NetBSD, qemu-arm, qemu-x86_64, xen other than linux/freebsd/windows which current LKL supports.
  • execuse: I haven't tested that much, only tested with qemu-arm, qemu-x86_64 on Linux.
• non hijack-based existing application support with a dedicated libc bind to LKL using musl libc
  • rumprun/frankenlibc unikernel integration
• new thread primitive based on green thread (a.k.a. userspace thread, related to official usermode thread support in LKL #250 ?)

The addition of rump hypercalls does not interfere the current implementation of LKL, but with additional external repositories (*2), you will run LKLed applications with rump hypercall.

Non-hijack application support requires to build an (existing) application by dedicated build toolchain (cross compilation as rumprun does), but it will solve most of hijack limitations such as conflicted symbols/namespaces, non exposed symbols (e.g., replace hidden symbols in glibc). With this toolchain, I confirmed nginx, netperf, and ghc (a haskell compiler) somehow work fine, though those need more work to be more than just a hello world.

The green thread implementation is obviously useful to avoid the expensive task of context switches. I saw some of netperf benchmarks outperform pthread based one (will re-measure with netperf in my spare time).
(rump hypercall has pthread-based thread implementation but I didn't test it in this PR).

I'm going to upstream the external repositories (*2) as well as this PR so, this PR will be a base of all of external dependencies.

I think merging non-master branch of LKL and synchronize periodically with master branch is also reasonable since the changes (of this PR) are a bit large.

*1 rump kernel
http://book.rumpkernel.org

*2 additional (external) repositories

• rumprun unikernel: https://github.com/libos-nuse/rumprun
• frankenlibc unikernel: https://github.com/libos-nuse/frankenlibc
• buildrump.sh scripts: https://github.com/libos-nuse/buildrump.sh
• musl libc LKL support: https://github.com/libos-nuse/musl

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