selftests/x86: Test signal frame XSTATE header corruption handling

This is very heavily based on some code from Thomas Gleixner.  On a system
without XSAVES, it triggers the WARN_ON():

  Bad FPU state detected at copy_kernel_to_fpregs+0x2f/0x40, reinitializing FPU registers.

  [ bp: Massage in nitpicks. ]

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Rik van Riel <riel@surriel.com>
Link: https://lkml.kernel.org/r/20210608144346.234764986@linutronix.de