Adaptation of siginfo fixup for the new bnd fields

Adaptation of siginfo fixup for the new bnd fields

New bnds fields will be always present for x86 architecture.
Fixup for compatibility layer 32bits has to be fixed.

It was added the nat_siginfo to serving as intermediate step
between kernel provided siginfo and the fix up routine.

When executing compat_siginfo_from_siginfo or
compat_x32_siginfo_from_siginfo first the buffer read from the kernel are
converted into the nat_signfo for homogenization, then the fields of
nat_siginfo are use to set the compat and compat_x32 siginfo fields.

In other to make this conversion independent of the system where gdb
is compiled the most complete version of the siginfo, named as native
siginfo, is used internally as an intermediate step.

Conversion using nat_siginfo is exemplified below:

compat_siginfo_from_siginfo or compat_x32_siginfo_from_siginfo:

buffer (from the kernel) -> nat_siginfo -> 32 / X32 siginfo
                      (memcpy)       (field by field)

siginfo_from_compat_x32_siginfo or siginfo_from_compat_siginfo:

32 / X32 siginfo -> nat_siginfo -> buffer (to the kernel)
          (field by field)    (memcpy)

Caveat: No support for MPX on x32.

2016-02-02  Walfred Tedeschi  <walfred.tedeschi@intel.com>

gdb/ChangeLog:

	* amd64-linux-siginfo.c (nat_siginfo_t, nat_sigval_t, nat_timeval):
	New types.
	(compat_siginfo): New bound fields added.
	(compat_x32_siginfo): New field added.
	(cpt_si_addr_lsb): New define.
	(compat_siginfo_from_siginfo): Use nat_siginfo.
	(siginfo_from_compat_siginfo): Use nat_siginfo.
	(compat_x32_siginfo_from_siginfo): Likewise.
	(siginfo_from_compat_x32_siginfo): Likewise.