AVR: Support new option -mprune-vectab.

This patch adds -mprune-vectab as a new target option.
It acts as a front end for the linker's --prune-vectab option
that has been added as https://sourceware.org/PR34679.
The compiler option has only an effect when the linker
actually supports --prune-vectab, which is tested for in
configure.ac.

gcc/
	* configure.ac [target=avr-*.*]: Test linker for --prune-vectab,
	and AC_DEFINE HAVE_LD_AVR_PRUNE_VECTAB if supported.
	* config.in: Rebuild.
	* configure: Rebuild.
	* config/avr/avr.opt (-mprune-vectab): New target option.
	* config/avr/specs.h (LINK_SPEC): Add %(link_prune_vectab).
	* config/avr/gen-avr-mmcu-specs.cc (print_mcu)
	[HAVE_LD_AVR_PRUNE_VECTAB]: Emit link_prune_vectab spec.
	* config/avr/avr.opt.urls: Rebuild.
	* doc/invoke.texi (AVR Options): Document -mprune-vectab.