signal: Add FPE_FLTUNK si_code for undiagnosable fp exceptions

Some architectures cannot always report accurately what kind of
floating-point exception triggered a floating-point exception trap.

This can occur with fp exceptions occurring on lanes in a vector
instruction on arm64 for example.

Rather than have every architecture come up with its own way of
describing such a condition, this patch adds a common FPE_FLTUNK
si_code value to report that an fp exception caused a trap but we
cannot be certain which kind of fp exception it was.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

arch/x86/kernel/signal_compat.c

@@ -26,7 +26,7 @@ static inline void signal_compat_build_tests(void)
 	 * new fields are handled in copy_siginfo_to_user32()!
 	 */
 	BUILD_BUG_ON(NSIGILL  != 11);
-	BUILD_BUG_ON(NSIGFPE  != 13);
+	BUILD_BUG_ON(NSIGFPE  != 14);
 	BUILD_BUG_ON(NSIGSEGV != 4);
 	BUILD_BUG_ON(NSIGBUS  != 5);
 	BUILD_BUG_ON(NSIGTRAP != 4);