[WebAssembly] Implement WASM_STACK_POINTER.

Use the .stack_pointer directive to implement WASM_STACK_POINTER for
specifying a global variable to be the stack pointer.

llvm-svn: 319797
diff --git a/llvm/test/CodeGen/WebAssembly/stack-alignment.ll b/llvm/test/CodeGen/WebAssembly/stack-alignment.ll
index 25e9d06..6128c8a 100644
--- a/llvm/test/CodeGen/WebAssembly/stack-alignment.ll
+++ b/llvm/test/CodeGen/WebAssembly/stack-alignment.ll
@@ -147,3 +147,5 @@
   call void @somefunc(i32* %static)
   ret void
 }
+
+; CHECK: .stack_pointer __stack_pointer