[SanitizerBinaryMetadata] Use weak __start_/__stop_ instead of dummy empty section

D130887 uses a dummy empty section `sanmd_covered` (with the SHF_GNU_RETAIN flag on
ELF) to prevent `undefined symbol: __start_sanmd_covered` if all `sanmd_covered`
are discarded by `ld --gc-sections` (in `-z start-stop-gc` mode).

The dummy `sanmd_covered` does not have the SHF_LINK_ORDER flag, so mixing it
with SHF_LINK_ORDER `sanmd_covered` causes an issue to GNU ld<2.36
(https://sourceware.org/bugzilla/show_bug.cgi?id=26256).

Similar to D98903 for SanitizerCoverage, let's make encapsulation symbols
undefined weak[1]. This additionally avoids size cost due to the dummy section and
symbol.

[1]: https://maskray.me/blog/2021-01-31-metadata-sections-comdat-and-shf-link-order

Reviewed By: melver

Differential Revision: https://reviews.llvm.org/D139276