Support building without PSEUDO-ATOMIC on POSIX safepoints

Support building without PSEUDO-ATOMIC on POSIX safepoints

  - Mark Lisp signal handlers with a flag `synchronous' to indicate
    whether we can (and must) handle them immediately.  Conversely,
    we understand this flag to imply a guarantee that the signal
    does not occur during allocation.

  - Any signal with a Lisp handler that is not synchronous is
    implemented in the runtime using a trampoline, which (instead of
    invoking Lisp code directly) first spawns a new pthread, which
    only then calls back into Lisp to invoke the handler function
    (with a fake signal context).

  - Used in particular for SIGINT.

  - For SIGPROF, introduce a second per-thread allocation region,
    which gets swapped with the usual region around the call into
    SIGPROF-HANDLER.  This handler is a special case, because it is
    careful not to trigger GC nor non-local unwinds, and we can
    safely return to the original region afterwards.

  - Add a new subclass SIGNAL-HANDLER-THREAD for this purpose,
    making it easy to identify these threads (e.g. in the test
    driver).

  - Run sprof tests while building the contrib.  Add a test stressing
    time profiling of allocation sequences.

Enable using :SB-SAFEPOINT-STRICTLY on features.

Quite usable already on x86 and x86-64; PPC still has more prominent
issues, e.g. in threads.impure.lisp.