doc/invoke.texi: CRIS: Remove references to cris-axis-linux-gnu

...and related options.  These stale bits were overlooked when support
for "Linux/GNU" and CRIS v32 was removed, before the gcc-11 release.

Resulting pdf, html and info inspected for sanity.

gcc:
	* doc/invoke.texi <CRIS>: Remove references to options for removed
	subtarget cris-axis-linux-gnu and tweak wording accordingly.