[WebAssembly] Remove StackPointerGlobal member variable from the driver.
[WebAssembly] Remove StackPointerGlobal member variable from the driver.

Since InputGlobal makes a copy of a given object, we can use a temporary
object allocated on the stack here.

Differential Revision: https://reviews.llvm.org/D43924

git-svn-id: f6089bf0e6284f307027cef4f64114ee9ebb0424@329337 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld