BM/cet: add signal/thread/legacy SHSTK and kernel IBT cases

[hongyuni]

Add the missing CET shadow stack cases into BM/cet:

• sig_stack.c: CET signal stack test, verify the signal-triggered function is protected by SHSTK (bare "a" access check and cp_test "s" violation).
• cet_thread.c: SHSTK violation in a new thread should be #CP blocked. Accept upstream SEGV_CPERR(3) in addition to the legacy si_code 8|10 so the case works on mainline kernels.
• glibc_shstk_test_legacy: build glibc_shstk_test.c without CET protection (NOCETFLAGS) and verify none of the violation params trigger #CP (no_cp).
• test_shadow_stack.c: treat an EINVAL from UFFDIO_REGISTER on a shadow stack VMA as SKIP, since recent kernels reject userfaultfd registration of special VMAs.

Kernel IBT b1/b2 cases:

• cet_tests.sh: kmod_ibt_illegal triggers a kernel-space missing-ENDBR #CP; block the case unless the kernel is booted with ibt=warn (and block on ibt=off) to avoid crashing the host.
• load_cet_driver block_tests with a clear message pointing at the required kernel-devel/headers when cet_ioctl.ko is missing.
• cet_driver/Makefile: fail early with an actionable error when the kernel build dir is absent, and suppress the module's missing-prototype/declaration warnings on -Werror kernels.
• cet_driver/cet_ioctl.h: guard CPUID_LEAF_XSTATE so it does not conflict with the kernel's own definition.

Document the ibt=warn requirement in the tests list and README, and wire the new binaries into Makefile, CMakeLists.txt and .gitignore.

(cherry picked from commit 8e9069c323d408fcd75b4b61ee669b8fe3fc5425)