[XRay][compiler-rt][Darwin] Use dynamic initialisation as an alternative

Summary:
In cases where we can't use the .preinit_array section (as in Darwin for
example) we instead use dynamic initialisation. We know that this
alternative approach will race with the initializers of other objects at
global scope, but this is strictly better than nothing.

Reviewers: kubamracek, nglevin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D40599

llvm-svn: 319366