Register --sysroot in the driver switches table

This change adjusts the processing of --sysroot to save the option in the
internal "switches" array, which lets self-specs test for it and provide a
default value possibly dependent on environment variables, as in

  --with-specs=%{!-sysroot*:--sysroot=%:getenv("WIND_BASE" /target)}

2021-12-20  Olivier Hainque  <hainque@adacore.com>

gcc/
	* gcc.c (driver_handle_option): do_save --sysroot.