This patch updates the siginfo_t struct definition to match the
definition from the kernel in here:

https://github.com/torvalds/linux/blob/master/include/uapi/asm-generic/siginfo.h

In particular, there are two main changes:

1. swap position of si_code and si_errno: si_code show come after
   si_errno in all systems except MIPS. Since we don't MIPS, the order
   is fixed for now, but can be easily \#ifdef'd if MIPS support is
   implemented in the future.

2. We add a union of structs that are filled depending on the signal
   raised.

This change was required for the fork and spawn integration tests in rv32,
since they fork/clone the running process, call wait/waitid/waitpid, and
read the status, which was wrong in rv32 because wait/waitid/waitpid are
implemented in rv32 using SYS_waitid.

SYS_waitid takes a pointer to a siginfo_t and fills the proper fields in
the struct. The previous siginfo_t definition was being incorrectly
filled due to not taking into account the signal raised.