BFD: Write Linux core PRSTATUS note into MIPS core file

BFD: Write Linux core PRSTATUS note into MIPS core file

On MIPS o32, n32 and n64 platforms information such as PID was not
correctly written into core file from GDB.

This fixes that behavior.

bfd/ChangeLog:

	* elf32-mips.c (elf32_mips_write_core_note): New function.
	(elf_backend_write_core_note): New macro.
	* elf64-mips.c (elf64_mips_write_core_note): New function.
	(elf_backend_write_core_note): New macro.
	* elfn32-mips.c (elf32_mips_write_core_note): New function.
	(elf_backend_write_core_note): New macro.