coroutine: Split qemu/coroutine-core.h off qemu/coroutine.h

qemu/coroutine.h and qemu/lockable.h include each other.

They need each other only in macro expansions, so we could simply drop
both inclusions to break the loop, and add suitable includes to files
that expand the macros.

Instead, move a part of qemu/coroutine.h to new qemu/coroutine-core.h
so that qemu/coroutine-core.h doesn't need qemu/lockable.h, and
qemu/lockable.h only needs qemu/coroutine-core.h.  Result:
qemu/coroutine.h includes qemu/lockable.h includes
qemu/coroutine-core.h.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20221221131435.3851212-5-armbru@redhat.com>
[Semantic rebase conflict with 7c10cb38 "accel/tcg: Add debuginfo
support" resolved]