selftests/nolibc: add basic infrastructure to ease creation of nolibc tests

This creates a "nolibc" selftest that intends to test various parts of
the nolibc component, both in terms of build and execution for a given
architecture.

The aim is for it to be as simple to run as a kernel build, by just
passing the compiler (for the build) and the ARCH (for kernel and
execution).

It brings a basic squeleton made of a single C file that will ease testing
and error reporting. The code will be arranged so that it remains easy to
add basic tests for syscalls or library calls that may rely on a condition
to be executed, and whose result is compared to a value or to an error
with a specific errno value.

Tests will just use a relative line number in switch/case statements as
an index, saving the user from having to maintain arrays and complicated
functions which can often just be one-liners.

MAINTAINERS was updated.

Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>