Use elfcore_write_linux_prstatus in gdb

Use elfcore_write_linux_prstatus in gdb

The gdb side.  Nothing fancy here, though it'd be easier to fill in
more fields now.  Targets that are known to need a custom
gdbarch_elfcore_write_linux_prstatus but don't have one yet can
install a stop-gap hook implementation that still calls into
elfcore_write_prstatus.  It is currently installed for MIPS n32 and
Sparc64 (though we're not writting any of pr_{u,s,cu,cs}time yet), and
x86-64 x32 too.

Signed-off-by: Pedro Alves <palves@redhat.com>