ELF object files can contain `.ctors` and `.dtors` sections that also
participate as initializers.
Reviewed By: lhames
Differential Revision: https://reviews.llvm.org/D154802
This patch drops the individual registration calls to the ObjC and Swift
runtimes (for selectors, classes, etc.), and instead creates a Mach header and
load commands that can be passed to _objc_map_images and _objc_load_images to
trigger registration and execution of +load methods. This approach supports
categories (for which there is no current registration API), and more closely
follows dyld's ObjC & Swift registration path.
