[WebAssembly] Import the linear memory and function table.

Instead of having .o files contain linear-memory and function table
definitions, use imports. This is more consistent with the stack pointer
being imported, and it's consistent with the linker being the one to
decide whether linear memory and function table are imported or defined
in the linked output. This implements tool-conventions #23.

Differential Revision: https://reviews.llvm.org/D40875

llvm-svn: 319989