stdio_native: initial import

[miri64]

Contribution description

This provides a stdio module for native, so the abstraction of stdio is the same as on other boards.

While implementing tests for #12975, I noticed, that on native when using vfs the fmt print functions would not work. This is because the fmt module uses the write() system call which is bend by the native_vfs module (or the newlib_syscalls_default module for real boards) to use vfs_write(). However, since native doesn't use a stdio module without this PR to print, but just writes to the hosts standard I/O directly, the STDIN, STDOUT, and STDERR are never initialized for vfs, vfs fails here:

RIOT/sys/vfs/vfs.c

Lines 320 to 323 in 33b1f9e

	int res = _fd_is_valid(fd);
	if (res < 0) {
	return res;
	}

Testing procedure

Without stdio_native (revert all commits related to it; the HEAD when opening), the provided test application tests/vfs_and_stdio will fail on native:

BOARD=native make -C tests/vfs_plus_stdio flash test

With stdio_native available it will succeed.

BOARD=native make -C examples/hello-world all term

and

BOARD=native make -C tests/shell all test

should also still work.

Issues/PRs references

None

[miri64]

Forgot to push the test 😅

[ghost]

ACK
test works as explained
PR is very reasonable
code looks good

[miri64]

@aabadie Can we merge?