aarch64: Improve --with-arch checks

aarch64: Improve --with-arch checks

- Check for invalid characters before further processing.  Allow only
  alphanumeric characters, "-", "+" and ".".
- Convert "." to "\." before using user input in a sed expression.
- Reject zero-length extension names.
- Quote variables used in echo commands, to avoid unwanted shell
  expansions.

Without these changes, various invalid inputs would be accepted, for
example due to misparsing of "*" and "." characters in regexps and
shell expansions.  Some inputs could also lead to an infinite loop.

gcc/ChangeLog:

	* config.gcc: Improve aarch64 --with-arch checks.