MDEV-34859: Failed to initialise non-blocking API

[knielsen]

Two patches to improve the portability and efficiency of the non-blocking library, especially on 64-bit ARM and/or OpenBSD.

First patch:

Implement native my_context for arm64 (aarch64). This is more
efficient than ucontext, and also makes the non-blocking API available
on arm64 platforms that do not have ucontext such as OpenBSD.

Second patch:

Implement boost::context as a fallback for non-blocking API support

The non-blocking API has native (assembler) implementations for x86_64,
i386, and (with recent patch) aarch64; these implementations are the most
efficient. For other architectures, a fallback to ucontext is supported.

But ucontext is not the most efficient, and it is not available on all
platforms (it has been deprecated in POSIX). The boost::context library
provides an alternative fallback that is available on more architectures and
should be more efficient than ucontext (if still not quite as fast as the
native support).

This patch adds a CMake option -DWITH_BOOST_CONTEXT=ON that adds
boost::context as a dependency of libmariadb to provide a fallback on
non-natively supported architectures. Boost::context is preferred over
ucontext when both are available.

The option is off by default and must be explicitly enabled by the
user. This avoids introducing a C++ dependency (including dependency
on a C++ compiler and on libstdc++) unless explicitly requested by the
user (libmariadb is otherwise C-only).