Merge #206

206: Update the Rust toolchain to nightly-2020-06-10. r=jrvanwhy a=jrvanwhy

This toolchain update renamed `asm` to `llvm_asm`, and should have the new `asm` syntax. I simply renamed `asm` to `llvm_asm` in this commit. I will send PRs later that refactor the system call interface and entry point; those PRs will replace the uses of `llvm_asm` with the new `asm`.

Co-authored-by: Johnathan Van Why <jrvanwhy@google.com>