BFD: Extract PID from MIPS core dump file

BFD: Extract PID from MIPS core dump file

On MIPS o32, n32 and n64 platforms, PID information was not
correctly propagated from core dump file to internal GDB
structures.  This patch fixes that behavior.

A correct PID is needed by `libthread_db' library supplied with
glibc repository revisions before commit c579f48edba8 ("Remove
cached PID/TID in clone") or released versions before 2.25 for
GDB to fetch value of TLS variable from core file.

bfd/ChangeLog:

	* elf32-mips.c (elf32_mips_grok_psinfo): Extract core->pid.
	* elf64-mips.c (elf64_mips_grok_psinfo): Likewise.
	* elfn32-mips.c (elf32_mips_grok_psinfo): Likewise.