[sanitizer_common] Add experimental flag to tweak dlopen(<main program>) (#71715)

This introduces an experimental flag 'test_only_replace_dlopen_main_program'. When enabled, this will replace dlopen(main program,...) with dlopen(NULL,...), which is the correct way to get a handle to the main program.

This can be useful when ASan is statically linked, since dladdr((void*)pthread_join) or similar will return the path to the main program.

Note that dlopen(main program,...) never ends well:
- PIE in recent glibc versions (glibc bugzilla 24323), or non-PIE: return an error
- PIE in current GRTE and older glibc: attempt to load the main program again, leading to reinitializing ASan and failing to remap the shadow memory.

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Co-authored-by: Thurston Dang <thurston@google.com>