[WebAssembly] Make __wasm_lpad_context thread-local

This makes `__wasm_lpad_context`, a struct that is used as a
communication channel between compiler-generated code and personality
function in libunwind, thread local. The library code will be changed to
thread local in the emscripten side.

Reviewed By: sbc100, tlively

Differential Revision: https://reviews.llvm.org/D119803