coroutine: add check for SafeStack in sigaltstack

Current implementation of LLVM's SafeStack is not compatible with
code that uses an alternate stack created with sigaltstack().
Since coroutine-sigaltstack relies on sigaltstack(), it is not
compatible with SafeStack. The resulting binary is incorrect, with
different coroutines sharing the same unsafe stack and producing
undefined behavior at runtime.

In the future LLVM may provide a SafeStack implementation compatible with
sigaltstack(). In the meantime, if SafeStack is desired, the coroutine
implementation from coroutine-ucontext should be used.
As a safety check, add a control in coroutine-sigaltstack to throw a
preprocessor #error if SafeStack is enabled and we are trying to
use coroutine-sigaltstack to implement coroutines.

Signed-off-by: Daniele Buono <dbuono@linux.vnet.ibm.com>
Message-id: 20200529205122.714-3-dbuono@linux.vnet.ibm.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>