Added Irix 5 support.

	Added Irix 5 support.
	* configure.in (mips-sgi-irix5*): New host and target.  Use irix5
	for both.
	* config/mips/irix5.mh, config/mips/irix5.mt,
	config/mips/xm-irix5.h, config/mips/nm-irix5.h,
	config/mips/tm-irix5.h, irix5-nat.c: New files for Irix 5 support.
	* mdebugread.c: New file, split out of mipsread.c.  Added
	elfmdebug_build_psymtabs routine.  Added some checks on external
	symbols.  Changed code to keep ecoff_debug_info and
	ecoff_debug_swap structs in the psymtab and in global pointers
	rather than retrieving them from the bfd.  Also changed to keep
	the pending list with the psymtab rather than the objfile (each
	psymtab for a single objfile points to the same pending list).
	* mipsread.c: Bulk of file moved into mdebugread.c, leaving just
	the sym_fns.
	* Makefile.in (SFILES): Added mdebugread.c.
	(OBS): Added mdebugread.o.
	(mdebugread.o): New target.
	* symfile.h: Declare mdebug_build_psymtabs and
	elfmdebug_build_psymtabs.
	* elfread.c (struct elfinfo): Added mdebugsect field.
	(elf_locate_sections): Remember location of .mdebug section.
	(elf_symfile_read): Call elfmdebug_build_psymtabs on .mdebug
	section.
	* infrun.c (AT_FUNCTION_START): Set to 0 if not already defined.
	(wait_for_inferior): Use AT_FUNCTION_START if it is defined to see
	if PC is at the start of a function.
	* mips-tdep.c (read_next_frame_reg): Use SIGFRAME_REG_SIZE, and
	give it a default definition.
	(mips_skip_prologue): Skip instructions which initialize $gp
	register.
	(in_sigtramp): New procedure, moved in from mipsread.c.
	* config/mips/tm-mips.h: Declare in_sigtramp.