[WebAssembly] Fix crash when relocation synbol is not live

When a symbol is GC'd it can still be references by relocations
in the debug sections, but such symbols are not assigned virtual
addresses.

This change adds a new global data symbol which gets GC'd but
should still appears in the output debug info, albeit with a 0
address.

Fixes 37555

Differential Revision: https://reviews.llvm.org/D47238

llvm-svn: 333047