fragments.texi, [...]: Update to reflect current (lack of) need for host configuration by hand.

	* doc/fragments.texi, doc/hostconfig.texi: Update to reflect
	current (lack of) need for host configuration by hand.

	* doc/gccint.texi, doc/rtl.texi, doc/tm.texi: Adjust cross
	references.  Documentation of some target macros moved from
	hostconfig.texi to tm.texi.

From-SVN: r49142