ASAN improvements

[maleadt]

• Moves the ASAN default options to the loader executable; this is the same as 1a82590, which got lost again after we introduced a loader executable
• This requires moving the sanitizer-related preprocessor definitions out of julia.h, which isn't included by the loader. I've put them into platform.h
• Additionally, define the NO_RTLD environment variable that's needed for LBT to work under ASAN. Not pretty, but makes a sanitized executable work out-of-the-box without having to set stuff in your environment.

May be worth backporting to 1.7 to get a better ASAN experience there.

[tkf]

I think we can simplify the CI script contrib/asan/ now a bit. It's useful for testing __asan_default_options works. I just pushed my suggestion directly to the PR. I tested locally and it works for me.

It looks like the CI result will show up here: https://buildkite.com/julialang/julia-master-experimental/builds/222

[DilumAluthge]

Yeah, if you want to see the CI logs for the asan Buildkite job, they'll be available here: https://buildkite.com/julialang/julia-master-experimental

[tkf]

I'm a bit hesitant to support backporting too many things in the middle of the release hustle. But maybe it can be treated as a bug fix? Anyway, if we backport this, #41522 should go in, too, for updating LBT.

[DilumAluthge]

I'm a bit hesitant to support backporting too many things in the middle of the release hustle.

We could always backport it to 1.7.1, right? Or any other future 1.7.x release?

[DilumAluthge]

Anyway, I'll take the backport label off for now.

[DilumAluthge]

@maleadt @vchuravy @tkf Are we going to backport this to 1.7, or no?

[maleadt]

No need, and it would require a backport of #41522 too.