Fix PR gdb/20287 - x32 and "gdb_static_assert (sizeof (nat_siginfo_t) == sizeof (siginfo_t))"

Building an x32 gdb trips on a static assertion:

  In file included from .../src/gdb/common/common-defs.h:71:0,
		   from .../src/gdb/nat/amd64-linux-siginfo.c:21:
  .../src/gdb/common/gdb_assert.h:26:66: error: size of array ‘never_defined_just_used_for_checking’ is negative
     extern int never_defined_just_used_for_checking[(expr) ? 1 : -1]
								    ^
  .../src/gdb/nat/amd64-linux-siginfo.c:113:1: note: in expansion of macro ‘gdb_static_assert’
   gdb_static_assert (sizeof (nat_siginfo_t) == sizeof (siginfo_t));
   ^

The problem is that the way nat_siginfo_t is defined, it can only
match the host's siginfo_t object when gdb is built as a 64-bit
program.

Several bits of nat_siginfo_t are off:

- nat_siginfo_t's _pad field's definition is:

   int _pad[((128 / sizeof (int)) - 4)];

  while /usr/include/bits/siginfo.h has:

   # define __SI_MAX_SIZE     128
   # if __WORDSIZE == 64
   #  define __SI_PAD_SIZE     ((__SI_MAX_SIZE / sizeof (int)) - 4)
   # else
   #  define __SI_PAD_SIZE     ((__SI_MAX_SIZE / sizeof (int)) - 3)
   # endif

  and __WORDSIZE == 32 for x32.  This is what causes the size of
  nat_siginfo_t to be wrong and the assertion to fail.

- the nat_clock_t type is incorrect for 64-bit.  We have this:

   /* For native 64-bit, clock_t in _sigchld is 64bit aligned at 4 bytes.  */
   typedef long __attribute__ ((__aligned__ (4))) nat_clock_t;

  however, /usr/include/bits/siginfo.h has:

   # if defined __x86_64__ && __WORDSIZE == 32
   /* si_utime and si_stime must be 4 byte aligned for x32 to match the
      kernel.  We align siginfo_t to 8 bytes so that si_utime and si_stime
      are actually aligned to 8 bytes since their offsets are multiple of
      8 bytes.  */
   typedef __clock_t __attribute__ ((__aligned__ (4))) __sigchld_clock_t;
   #  define __SI_ALIGNMENT __attribute__ ((__aligned__ (8)))
   # else
   typedef __clock_t __sigchld_clock_t;
   #  define __SI_ALIGNMENT
   # endif

  So we're currently forcing 4-byte alignment on clock_t, when it
  should only be so for x32, not 64-bit.

The fix:

 - Leaves nat_siginfo_t strictly for the 64-bit ABI.

 - Adds a new typedef for the siginfo type that ptrace uses
   (ptrace_siginfo_t).  An x32 gdb always gets/sets an x32 siginfo_t
   type with PTRACE_GETSIGINFO/PTRACE_SETSIGINFO.

 - Uses this new ptrace_siginfo_t type instead of nat_siginfo_t as the
   intermediate conversion type.

gdb/ChangeLog:
2016-07-26  Pedro Alves  <palves@redhat.com>

	* amd64-linux-nat.c (amd64_linux_siginfo_fixup): Rename 'native'
	parameter to 'ptrace'.
	* nat/amd64-linux-siginfo.c (GDB_SI_SIZE): New define.
	(nat_uptr_t): New an unsigned long.
	(nat_clock_t): Remove attribute __aligned__.
	(struct nat_timeval): Delete.
	(nat_siginfo_t): Remove attribute __aligned__.
	(ptrace_siginfo_t): Define.
	(compat_siginfo_from_siginfo, siginfo_from_compat_siginfo)
	(compat_x32_siginfo_from_siginfo)
	(siginfo_from_compat_x32_siginfo): Make 'from' parameter const.
	Convert through a ptrace_siginfo_t instead of a nat_siginfo_t.
	Remove casts.
	(amd64_linux_siginfo_fixup_common): Rename 'native' parameter to
	'ptrace'.  Remove static assertions.
	(top level): New static assertions.

gdb/gdbserver/ChangeLog:
2016-07-26  Pedro Alves  <palves@redhat.com>

	* linux-x86-low.c (x86_siginfo_fixup): Rename 'native' parameter
	to 'ptrace'.

gdb/amd64-linux-nat.c

@@ -321,25 +321,25 @@ ps_get_thread_area (const struct ps_prochandle *ph,
 }
 
 
-/* Convert a native/host siginfo object, into/from the siginfo in the
+/* Convert a ptrace/host siginfo object, into/from the siginfo in the
    layout of the inferiors' architecture.  Returns true if any
    conversion was done; false otherwise.  If DIRECTION is 1, then copy
-   from INF to NATIVE.  If DIRECTION is 0, copy from NATIVE to
+   from INF to PTRACE.  If DIRECTION is 0, copy from PTRACE to
    INF.  */
 
 static int
-amd64_linux_siginfo_fixup (siginfo_t *native, gdb_byte *inf, int direction)
+amd64_linux_siginfo_fixup (siginfo_t *ptrace, gdb_byte *inf, int direction)
 {
   struct gdbarch *gdbarch = get_frame_arch (get_current_frame ());
 
   /* Is the inferior 32-bit?  If so, then do fixup the siginfo
      object.  */
   if (gdbarch_bfd_arch_info (gdbarch)->bits_per_word == 32)
-      return amd64_linux_siginfo_fixup_common (native, inf, direction,
+      return amd64_linux_siginfo_fixup_common (ptrace, inf, direction,
 					       FIXUP_32);
   /* No fixup for native x32 GDB.  */
   else if (gdbarch_addr_bit (gdbarch) == 32 && sizeof (void *) == 8)
-      return amd64_linux_siginfo_fixup_common (native, inf, direction,
+      return amd64_linux_siginfo_fixup_common (ptrace, inf, direction,
 					       FIXUP_X32);
   else
     return 0;