[gdb/doc] Remove references to no-longer-supported systems

[gdb/doc] Remove references to no-longer-supported systems

HP-UX and SGI/IRIX are no longer supported.  Remove references
throughout.

AFAICS from the sources, "catch fork" seems to be supported in
multiple Unix systems -- just remove the "only works on xxx" remarks.

Update the list of supported shared library types.

gdb/doc/ChangeLog:

	* gdb.texinfo (Threads): Remove mention of SGI.
	(Forks): Remove mention of HP-UX.
	(Breakpoints): Remove mention of HP-UX.
	(Set Watchpoints) <hardware watchpoints>: Don't mention HP-UX.
	Reword in terms of architectures.
	(Set Catchpoints) <catch exec, catch fork, catch vfork>: Don't
	mention supported systems.
	(Convenience Vars): Don't mention HP-UX.
	(Jumping): Remove mention of HP-UX in comment.
	(Files) <shared libraries>: Update supported shared library types
	list.  Remove mention of HP-UX.
	(Native): Remove HP-UX subsection.
	(SVR4 Process Information): Remove mention of HP-UX.