i#2659 syscall restart: switch to using SA_RESTART

[derekbruening]

Changes the strategy for handling interrupted syscalls to use SA_RESTART,
which adds easy handling of interruptions of auto-restart syscalls in
native code, such as during attach or in client C code.

This also simplifies auto-restart of app syscalls, as we now have foolproof
identification of auto-restart situations (before our syscall-number-based
check was inaccurate) and the kernel has restored the clobbered register
value for us. That eliminates the TLS store we were doing on ARM and the
decode on x86.

For fragment-inlined syscalls, we don't need to do anything anymore: we
deliver the signal immediately and the kernel has already set up the proper
resumption state.

For gencode syscalls, for sane post-syscall handling, we leverage i#1145's
auto-restart emulation and undo what the kernel did. This lets us go back
to dispatch for a clean sequence of events.

Updates synch and translation checks for post-syscall to also account for
syscall-restart locations.

For sysenter, this change means we now see threads at the int 0x80 restart
point whose fall-through is our vsyscall hook. To avoid interpreting our
own hook we map that PC to our displaced code, and mapping back in
dr_fragment_app_pc().

Fixes a race between going native and unhooking the vsyscall (to avoid
takeover problems) by redirecting threads at vsyscall or the int 0x80
restart to our own gencode.

Adds a test of a client making a blocking auto-restart syscall to ensure it
is not terminated with EINTR. This was not simple to arrange in a non-racy
manner and required adding a new feature of immediately delivering a signal
that arrives in DR or client code where the receiving thread is at a safe
point (because if we delay, the test cannot ensure the signal was received).

Fixes #2659

[derekbruening]

appveyor is api.detach #2246