Closes #2617

Closes #3302

jit: Clean rustllvm code, let rustc expose __morestack instead of linking in libmorestack and return _rust_main and call it from rustc

This commit may require changes to the following Servo files:
 * rust-geom/matrix.rs
 * servo/platform/osmain.rs
 * rust-layers/layers.rs
 * rust-geom/matrix.rs