Replaces remaining uses of x86_64-unknown-hermitkernel

CI: Explicitly set build-std without std
Else, cargo unifies with rusty-hermit's outer .cargo/config.toml and
tries to build std for the kernel, which fails with:

error[E0432]: unresolved import `crate::sys::hermit::thread_local_dtor`
 --> /home/mkroening/.rustup/toolchains/nightly-2021-10-20-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/sys/hermit/thread.rs:9:25
  |
9 | use crate::sys::hermit::thread_local_dtor::run_dtors;
  |                         ^^^^^^^^^^^^^^^^^ could not find `thread_local_dtor` in `hermit`

error[E0432]: unresolved import `crate::sys::hermit::thread_local_dtor`
   --> /home/mkroening/.rustup/toolchains/nightly-2021-10-20-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/sys/hermit/mod.rs:122:29
    |
122 |     use crate::sys::hermit::thread_local_dtor::run_dtors;
    |                             ^^^^^^^^^^^^^^^^^ could not find `thread_local_dtor` in `hermit`

I noticed this due to failing docs CI [1]
The rename began in 18dd15f5.
It seems that the repository-local json target specifications are not used, so I removed those.

[1] https://github.com/hermitcore/libhermit-rs/runs/2889060428

- explicit activation of optimized core functions (e.g. memcpy) isn't longer required

This reverts commit 8f83d097.

- don't rename symbol for newlib

afterwards `cargo build` without any additional flag builds
libhermit-rs for x86_64

Adds an initial version of integration tests. The tests are located in
the top level of the 'tests' directory. Integration tests are executed via
the hermit_test_runner.py script, which can use either uhyve or qemu to execute
the tests.
In this initial version, using the test_harness is deactivated.