Integrate BoringSSL shim

The BoringSSL test suite contains numerous tests which OpenSSL does not.

The BoringSSL test runner uses a shim to launch the library and execute the
tests. This is a version of the BoringSSL shim converted to compile against
OpenSSL instead.

This is primarily based on the work of David Benjamin from the BoringSSL
project who did most of the necessary conversion. It also includes a few
other tweaks for opacity changes etc.

This is based on a *very* old version of BoringSSL from commit f277add6c.
That was the last commit known to work with this patched shim. Later
versions may also work but lots of merge conflicts occur when trying to
bring it up to date.

At the moment this has not been integrated into the build system. There is
a very simple standalone makefile in the ossl_shim directory which should
be executed directly before tyring to use the shim.

Reviewed-by: Richard Levitte <levitte@openssl.org>