S390: Add support for core dump NOTE sections

S390: Add support for core dump NOTE sections

This enhances the 32-bit and 64-bit s390 ELF backends with support for
reading and writing the core dump note sections NT_PRSTATUS and
NT_PRPSINFO.  Byte swapping is done as appropriate, such that core files
can now be processed correctly on non-s390 platforms.

bfd/ChangeLog:

	* elf32-s390.c (stdarg.h): New include.
	(elf_s390_grok_psinfo): New function.
	(elf_s390_write_core_note): New function.
	(elf_backend_grok_psinfo): Declare backend hook.
	(elf_backend_write_core_note): Likewise.
	* elf64-s390.c (stdarg.h): New include.
	(elf_s390_grok_prstatus): New function.
	(elf_s390_grok_psinfo): New function.
	(elf_s390_write_core_note): New function.
	(elf_backend_grok_prstatus): Declare backend hook.
	(elf_backend_grok_psinfo): Likewise.
	(elf_backend_write_core_note): Likewise.