fail to build demo example in latest nightly

[maxtnuk]

when i build demo with latest nightly rust, i got build fail like this

Caused by:
  process didn't exit successfully: `/home/maxtnt/workspace/uni_kernel_test/target/debug/build/hermit-sys-13f6625d237250fc/build-script-build` (exit code: 101)
  --- stdout
  cargo:rerun-if-env-changed=HERMIT_IP
  cargo:rerun-if-env-changed=HERMIT_GATEWAY
  cargo:rerun-if-env-changed=HERMIT_MASK
  Build libhermit-rs output-status: exit code: 101
  Build libhermit-rs output-stdout: 
  Build libhermit-rs output-stderr:     Updating crates.io index
      Blocking waiting for file lock on package cache
  error: Package `test v0.0.0 (/home/maxtnt/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/test)` does not have the feature `compiler-builtins-asm`

it looks like problem with compiler-builtins-asm feature in test mod
I found that compiler-builtins-asm feature is removed in nightly-2021-01-22(1.51.0)
check out this commit
when i downgrade the nightly version, it works!!

so i think we need specify nightly version in toolchain or modify source code

[stlankes]

Thanks for the hint. I knew that they wanted to change the default behavior. I was not aware that this has already happened. I will fix it.

[stlankes]

I think that #110 solves this issue.

[maxtnuk]

Thanks!!