[LibOS] Rework signal handling and syscall emulation

Change log (most important only):
- unify CPU context structures - now we have only one version -
  `PAL_CONTEXT` - which is shared between LibOS and PALs and it should
  depend only on the host architecture (not OS),
- syscalls emulation changed:
  - dedicated LibOS stack is now used for syscalls emulation,
  - removed one indirection level in syscalls table - now it stores
    `shim_do_*` functions directly,
- signal handling - completely rewritten:
  - all signal queues use proper locking schemes now,
  - signals are handled *only* when returning to the user app from LibOS
    or PAL,
  - nested signals are now possible,
  - the app is allowed to jump out of signal handler with the same
    sematics as on normal Linux,
  - signal altstack is now fully supported,
  - syscall restarting is now supported,
  - doing a backtrace from the signal handler works properly,
- disallow injecting host-level signals, with one exception, see
  `sys.enable_sigterm_injection` manifest option for more details.

Examples/python-simple/python.manifest.template

@@ -21,6 +21,9 @@ loader.env.PYTHONHOME = "$(PYTHONHOME)"
 loader.env.PYTHONPATH = "$(PYTHONHOME):$(PYTHONHOME)/plat-$(ARCH_LONG):$(PYTHONDISTHOME):$(PYTHONHOME)/lib-dynload"
 loader.env.HOME = "/home/user"
 
+# Allow for injecting SIGTERM signal from the host.
+sys.enable_sigterm_injection = 1
+
 # Mounted FSes. The following "chroot" FSes mount a part of the host FS into the
 # guest. Other parts of the host FS will not be available in the guest.