Auto merge of #4950 - alexcrichton:rustflags-orderd, r=matklad

Ensure `[target]` rustflags are deterministically passed

The usage of `HashMap` in the `Config` tables introduced some nondeterminism, so
reverse that with a sort right before we pass it down to rustc. One day we'll
probably want to sort by the position where these keys were defined, but for now
a blanket sort should do the trick.

Closes #4935