configure: add flags to support SafeStack

This patch adds a flag to enable/disable the SafeStack instrumentation
provided by LLVM.

On enable, make sure that the compiler supports the flags, and that we
are using the proper coroutine implementation (coroutine-ucontext).
On disable, explicitly disable the option if it was enabled by default.

While SafeStack is supported only on Linux, NetBSD, FreeBSD and macOS,
we are not checking for the O.S. since this is already done by LLVM.

Signed-off-by: Daniele Buono <dbuono@linux.vnet.ibm.com>
Message-id: 20200529205122.714-4-dbuono@linux.vnet.ibm.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>