util/coroutine: Cleanup start_switch_fiber_ for TSAN.

util/coroutine: Cleanup start_switch_fiber_ for TSAN.

This is a cleanup patch to follow-up the patch which introduced TSAN.
This patch makes separate start_switch_fiber_ functions for TSAN and ASAN.

This does two things:
1. Unrelated ASAN and TSAN code is separate and each function only
   has arguments that are actually needed.
2. The co->tsan_caller_fiber and co->tsan_co_fiber fields are only
   access from within #ifdef CONFIG_TSAN.

Signed-off-by: Robert Foley <robert.foley@linaro.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20200626170001.27017-1-robert.foley@linaro.org>
Message-Id: <20200701135652.1366-5-alex.bennee@linaro.org>