Update 'threads-xform' for LLVM 9

[alexcrichton]

In LLVM 9 LLD has been updated to emit shared memory and passive
segments by default for threaded code, and __wasm_init_memory is a
function exported used to initialize memory. Update our
transform/runtime here to hook up all those wires correctly.

Closes #1631

[alexcrichton]

Note that this doesn't actually update the compiler used to produce the raytrace-parallel example just yet. This will require rust-lang/rust changes at rust-lang/rust#62809 to make it so it's not too burdensome to produce it here, but once that lands only a small patch will be necessary to update the example's nightly compiler.