linux(4): Rework the definition of struct siginfo to match Linux actual one

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.

Move struct siginfo definition under /compat/linux to reduce MD part.
To avoid headers polution include linux_siginfo.h in the MD linux.h

MFC after: 2 weeks