[SanitizerBinaryMetadata] Make module_[cd]tor external

If a COMDAT key has a local linkage, it behaves as `comdat nodeduplicate` and
llvm/lib/Linker/LinkModules.cpp does not deduplicate its members.
This is not intended. Switch to an external linkage to allow deduplication.

See also https://maskray.me/blog/2021-07-25-comdat-and-section-group#grp_comdat

Reviewed By: melver

Differential Revision: https://reviews.llvm.org/D143530