This commit adds the newlib retargetable locking interface function
implementations in order to make newlib functions thread safe.

The newlib retargetable locking interface is internally called by the
standard C library functions provided by newlib to synchronise access
to the internal shared resources.

By default, the retargetable locking interface functions defined within
the newlib library are no-op. When multi-threading is enabled (i.e.
`CONFIG_MULTITHREADING=y`), the Zephyr-side retargetable locking
interface implementations override the default newlib implementation
and provide locking mechanism.

The retargetable locking interface may be called with either a static
(`__lock__...`) or a dynamic lock.

The static locks are statically allocated and initialised immediately
after kernel initialisation by `newlib_locks_prepare`.

The dynamic locks are allocated and de-allocated through the
`__retargetable_lock_init[_recursive]` and
`__retarget_lock_close_[recurisve]` functions as necessary by the
newlib functions. These locks are allocated in the newlib heap using
the `malloc` function when userspace is not enabled -- this is safe
because the internal multi-threaded malloc lock implementations
(`__malloc_lock` and `__malloc_unlock`) call the retargetable locking
interface with a static lock (`__lock__malloc_recursive_mutex`). When
userspace is enabled, the dynamic locks are allocated and freed through
`k_object_alloc` and `k_object_release`.

Note that the lock implementations used here are `k_mutex` and `k_sem`
instead of `sys_mutex` and `sys_sem` because the Zephyr kernel does not
currently support dynamic allocation of the latter. These locks should
be updated to use `sys_mutex` and `sys_sem` when the Zephyr becomes
capable of dynamically allocating them in the future.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>

This commit adds the `static` keyword to the test functions that are
not intended to be globally available.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>

This commit adds the tests for the newlib retargetable locking
interface, as well as the tests for the internal lock functions that
are supposed to internally invoke the retargetable locking interface.

All of these tests must pass when the toolchain newlib is compiled with
the `retargetable-locking` and `multithread` options, which are
required to ensure that the newlib is thread-safe, enabled. If the
toolchain newlib is compiled with either of these options disabled,
this test will fail.

This commit also adds the userspace testcases to ensure that the newlib
is thread-safe in the user mode.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>

In order to use the newlib retagetable locking interface (for thread
safety), which requires the newlib multi-threading feature to be
enabled, the Zephyr SDK 0.13.0 or above must be used.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>