[lld][WebAssembly] Add suppport for PIC + passive data initialization

This change improves our support for shared memory to include
PIC executables (and shared libraries).

To handle this case the linker-generated `__wasm_init_memory`
function (that only exists in shared memory builds) must be
capable of loading memory segements at non-const offsets based
on the runtime value of `__memory_base`.

Differential Revision: https://reviews.llvm.org/D92620