[WebAssembly] Add placeholders for R_WASM_TABLE_INDEX_REL_SLEB relocations
Previously in the object format we punted on this and simply wrote
zeros (and didn't include the function in the elem segment). With
this change we write a meaningful value which is the segment
relative table index of the associated function.
This matches the that wasm-ld produces in -r mode. This inconsistency
between the output the MC object writer and the wasm-ld object
writer could cause warnings to be emitted when reading back in the
output of wasm-ld -r. See:
This only applies to this one relocation type which is only generated
when compiling in PIC mode.
Differential Revision: https://reviews.llvm.org/D80774