remove set_fs calls from the coredump code v4

[Christoph Hellwig]

Hi all,

this series gets rid of playing with the address limit in the exec and
coredump code.  Most of this was fairly trivial, the biggest changes are
those to the spufs coredump code.

Changes since v3:
 - fix x86 compilation with x32 in the new version of the signal code
 - split the exec patches into a new series

Changes since v2:
 - don't cleanup the compat siginfo calling conventions, use the patch
   variant from Eric with slight coding style fixes instead.

Changes since v1:
 - properly spell NUL
 - properly handle the compat siginfo case in ELF coredumps