devel/wasi-compiler-rt11, wasi-libc, wasi-libcxx: New ports for WebAssembly System Interface (WASI) libc, clang builtins (compiler-rt), libc++, libcxxabi

WASI is a "syscall" interface for WebAssembly, significantly inspired by CloudABI.
It is supported in clang 8 and up, but to actually build C programs,
it needs clang builtins and the libc/sysroot. And for C++, also libc++ and libcxxabi.

Submitted by: Greg V <greg%unrelenting.technology>
Differential Revision: https://reviews.freebsd.org/D22960