selftests/x86: Add tests for signal frame FPU portability

Add a new selftest tools/testing/selftests/x86/sigframe_fpu_portability.c
to verify signal frame portability and consistency when the xstate size
is shrunk:

- test_valid_shrunk_xstate_size: Verifies that the kernel correctly
  restores the xstate context from a signal frame where xstate_size has
  been manually shrunk to only cover active features, as long as the
  FP_XSTATE_MAGIC2 marker is correctly placed. This simulates migrating
  a process created on a host with fewer xstate features to a host with
  more features.

- test_invalid_shrunk_xstate_size: Verifies that the kernel rejects
  (via SIGSEGV) a signal frame where xstate_size is smaller than required
  by the enabled features in the xfeatures mask.

Signed-off-by: Andrei Vagin <avagin@google.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Alexander Mikhalitsyn <alexander@mihalicyn.com>
Reviewed-by: Chang S. Bae <chang.seok.bae@intel.com>
Link: https://patch.msgid.link/20260925162454.1403405-8-avagin@google.com

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