install.texi: Update GCN's newlib requirements

install.texi: Update GCN's newlib requirements

GCN requires 4.3.0 or newer, but improvements and bug fixes were added
in later versions; before 4.3.0 was listed as required and several versions
up to post-4.5.0 commits were mentioned. Now that 4.6.0 has been released,
just referring to 4.6.0 is sufficient. As fixes are important and there is no
real reason to use an older Newlib: Only list 4.6.0+ as to be used,
instead of also mentioning 4.3.0+ (and other older releases).

gcc/ChangeLog:

	* doc/install.texi (gcn): Require Newlib 4.6.0+, replacing 4.3.0+
	requirement with long list of recommended fixes up to post-4.5.0.