linux: Add openat2 (BZ 31664)

linux: Add openat2 (BZ 31664)

It was added on Linux 5.6, as an extension of openat.  Different
than other open-like function, the kernel only provides the LFS
variant (so files larger than 4GB always succeed, as other functions
with offset larget than off_t).  Similar to other open function,
the new symbol is a cancellable entrypoint.

The testcase added only stress some of the syscall provided
functionalities and it is based on existent kernel selftest.  Since
the prototype does not use variadic arguments, there is no need to
add fortify wrapper to catch wrong usages.

Checked on x86_64-linux-gnu.