[NFCI] Move SANITIZER_WEAK_IMPORT to sanitizer_common (#79208)

SANITIZER_WEAK_IMPORT is useful for any call that needs to be
conditionally linked in. This is currently used for the
tsan_dispatch_interceptors, but can be used for other calls introduced
in newer versions of MacOS. (such as `aligned_alloc` in this PR
https://github.com/llvm/llvm-project/pull/79198).

This PR moves the definition to a higher level so it can be used in
other sanitizers.