signal: Remove the need for __ARCH_SI_PREABLE_SIZE and SI_PAD_SIZE

Rework the defintion of struct siginfo so that the array padding
struct siginfo to SI_MAX_SIZE can be placed in a union along side of
the rest of the struct siginfo members.  The result is that we no
longer need the __ARCH_SI_PREAMBLE_SIZE or SI_PAD_SIZE definitions.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

arch/s390/include/uapi/asm/Kbuild

@@ -18,3 +18,4 @@ generic-y += shmbuf.h
 generic-y += sockios.h
 generic-y += swab.h
 generic-y += termbits.h
+generic-y += siginfo.h