selftests/x86: Update map_shadow_stack syscall nr

Shadow stack's selftest utilizes the map_shadow_stack syscall. The
syscall is new with the feature, but the selftests cannot automatically
find the headers for the kernel source tree they are located in. This
resulted in the shadow stack test failing to build until the brand new
headers were installed.

To avoid this, a copy of the new uapi defines needed by the test were
included in the selftest (see link for discussion). When shadow stack was
merged the syscall number was changed, but the copy in the selftest was
not updated.

So update the copy of the syscall number define used when the required
headers are not installed, to have the final syscall number from the
merge.

Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/lkml/Y%2FijdXoTAATt0+Ct@zn.tnic/
Link: https://lore.kernel.org/r/20230901181652.2583861-1-rick.p.edgecombe@intel.com
Reviewed-by: Sohil Mehta <sohil.mehta@intel.com>
Fixes: df57721f9a63 ("Merge tag 'x86_shstk_for_6.6-rc1' of [...]")

selftests/x86: Add shadow stack test

Add a simple selftest for exercising some shadow stack behavior:
- map_shadow_stack syscall and pivot
- Faulting in shadow stack memory
- Handling shadow stack violations
- GUP of shadow stack memory
- mprotect() of shadow stack memory
- Userfaultfd on shadow stack memory
- 32 bit segmentation
- Guard gap test
- Ptrace test

Co-developed-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Tested-by: Pengfei Xu <pengfei.xu@intel.com>
Tested-by: John Allen <john.allen@amd.com>
Tested-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/all/20230613001108.3040476-40-rick.p.edgecombe%40intel.com