[WebAssembly] Use __stack_pointer global when writing wasm binary

[WebAssembly] Use __stack_pointer global when writing wasm binary

This ensures that symbolic relocations are generated for stack
pointer manipulations.

These relocations are of type R_WEBASSEMBLY_GLOBAL_INDEX_LEB.
This change also adds support for reading relocations of this
type in WasmObjectFile.cpp.

Since its a globally imported symbol this does mean that
the get_global/set_global instruction won't be valid until
the objects are linked that global used in no longer an
imported global.

Differential Revision: https://reviews.llvm.org/D34172

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@305616 91177308-0d34-0410-b5e6-96231b3b80d8