i#1979 Mac64: small app support on Mojave

[derekbruening]

Fixes several issues to get small 64-bit apps to run on Mojave:

• Fixes library discovery problems with the dyld shared cache.
  On Mojave, SYS_shared_region_check_np returns the whole lib region,
  while the existing code assumed its return value was the shared
  __LINKEDIT segment.

• Demotes a DOCHECK at level 1 in check_thread_vm_area to 3 for
  Mac64 as it shows up as a large performance hit apparently due
  to slow memory queries.

• Segment fixes: fixes two issues hit due to DR and lib segments being
  equal: a bb building assert for -no_mangle_app_seg and a
  dr_insert_get_seg_base() assert.

• Attempts to reduce overhead in memquery_from_os() where it needs extra
  iterations to find the surrounding bounds of a free area, by walking
  backward instead of starting from 0.

• Uses SYS_stat64 and SYS_fstat64 for stat syscalls.

• Basic signal support:

  • dynamorio_sigreturn() syscall number.
  • master_signal_handler() setup to invoke sigreturn with the proper arguments.
  • Accessing the frame within master_signal_handler_C().
  • No support yet for AVX512 but put some pieces in place and comments for
    future work.
  • fixup_rtframe_pointers() and copy_frame_to_stack().
  • execute_handler_from_dispatch(): set up arguments for app handler,
    including the style.
  • handle_sigreturn(): obtain signal number from siginfo.

Issue: #1979

[derekbruening]

Appveyor failure is client.thread last_lock assert: #3560.