install.texi (Configuration): Document the --with-long-double-format={ibm,ieee} PowerPC configuration options.

2018-02-07  Michael Meissner  <meissner@linux.vnet.ibm.com>

	* doc/install.texi (Configuration): Document the
	--with-long-double-format={ibm,ieee} PowerPC configuration
	options.

From-SVN: r257473