Move mips_refhi_list to bfd tdata

Move mips_refhi_list to bfd tdata

Similar to commit c799eddb3512, but for mips-ecoff.  mips-ecoff is
marked obsolete, but we still allow reading of these object files in
a number of mips targets.

	* coff-mips.c (struct mips_hi, mips_refhi_list): Delete.
	(mips_refhi_reloc, mips_reflo_reloc): Access mips_refhi_list
	in ecoff_data.
	* ecoff.c (_bfd_ecoff_close_and_cleanup): New function.
	* libecoff.h (struct mips_hi): Moved from coff-mips.c.
	(struct ecoff_tdata): Add mips_refhi_list.
	(_bfd_ecoff_close_and_cleanup): Declare.