oeqa/selftest: add new test for toolchain switching

oeqa/selftest: add new test for toolchain switching

Add a basic test for the toolchain switching code: set the
toolchain to GCC by default but Clang for a specific recipe, and verify
that two recipes are built with the expected compiler.

This works because before we strip the installed binaries there is a
.comment segment that contains the list of toolchains used.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>