Changes to the b_sanitize option

Before 1.8 the b_sanitize option was a combo option, which is an enumerated set of values. In 1.8 this was changed to a free-form array of options where available sanitizers are not hardcoded anymore but instead verified via a compiler check.

This solves a number of longstanding issues such as:

• Sanitizers may be supported by a compiler, but not on a specific platform (OpenBSD).
• New sanitizers are not recognized by Meson.
• Using sanitizers in previously-unsupported combinations.

To not break backwards compatibility, calling get_option('b_sanitize') continues to return the configured value as a string, with a guarantee that address,undefined remains ordered.