borysp c24bddd5aa [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.

2021-02-05 14:11:21 +01:00

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