x86: shadow stack support

[rppt]

Shadow stack support for userspace finally made it to the kernel and varying level of success to glibc.

This PR enables shadow stack support in CRIU.

Aside from saving/restoring the actual shadow stack contents and control, there are some changes to the way CRIU calls rt_sigreturn and a bit of black magic around restoring of the shadow stack contents.

As it's still unclear what will be glibc policy about making shadow stack on or off by default, this patchset takes care of both cases and lets CRIU fully control shadow stack for the restored tasks.

Testing:

• kernel v6.6 or later
• glibc from https://gitlab.com/x86-glibc/glibc/-/tree/users/hjl/cet/v9/master
• to run zdtm tests with shadow stack enabled:

cd /path/to/criu
USERCFLAGS="-fcf-protection" make -C test/zdtm  -j$(nproc)
export GLIBC_TUNABLES=glibc.cpu.hwcaps=SHSTK

[codecov-commenter]

Codecov Report

Attention: 74 lines in your changes are missing coverage. Please review.

Comparison is base (b17a73b) 70.62% compared to head (4b86398) 70.17%.

:exclamation: Current head 4b86398 differs from pull request most recent head 394dab0. Consider uploading reports for the commit 394dab0 to get more accurate results

Files	Patch %	Lines
criu/arch/x86/shstk.c	21.25%	63 Missing :warning:
criu/kerndat.c	46.66%	8 Missing :warning:
criu/mem.c	50.00%	2 Missing :warning:
criu/proc_parse.c	80.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## criu-dev #2306 +/- ## ============================================ - Coverage 70.62% 70.17% -0.45% ============================================ Files 134 135 +1 Lines 33316 34153 +837 ============================================ + Hits 23528 23968 +440 - Misses 9788 10185 +397 ```

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[github-actions[bot]]

A friendly reminder that this PR had no activity for 30 days.

[avagin]

Sorry for the delay. It is still in my todo list. @0x7f454c46 @mihalicyn, you help will be welcome too;)

[avagin]

Merged. Thanks a lot.