[lld][WebAssembly] Delay the merging of data section when dynamic linking

With dynamic linking we have the current limitation that there can be
only a single active data segment (since we use __memory_base as the
load address and we can't do arithmetic in constant expresions).

This change delays the merging of active segments until a little later
in the linking process which means that the grouping of data by section,
and the magic __start/__end symbols work as expected under dynamic
linking.

Differential Revision: https://reviews.llvm.org/D96453