AVR: Update weblinks to AVR-LibC.

AVR: Update weblinks to AVR-LibC.

AVR-LibC has moved to GitHub, adjust web links:
https://github.com/avrdudes/avr-libc (project)
https://avrdudes.github.io/avr-libc/avr-libc-user-manual (wwwdocs)

gcc/
	* doc/invoke.texi (AVR Options): Update AVR-LibC weblink from
	nongnu.org to https://github.com/avrdudes/avr-libc
	* doc/extend.texi (AVR Named Address Spaces): Same.
	(AVR Function Attributes): Same.
	* doc/install.texi (Cross-Compiler-Specific Options, AVR): Same.