[WebAssembly] Don't generate unused table entries.

When generating PIC output only relocations of type
R_WASM_TABLE_INDEX_REL_SLEB should generate table entries.

R_WASM_TABLE_INDEX_I32 get resolved at runtime via the auto-generated
__wasm_apply_relocs functions.

R_WASM_TABLE_INDEX_SLEB are not allowed in PIC code.

Differential Revision: https://reviews.llvm.org/D61539

llvm-svn: 360165