[Clang][OpenMP Offload] Move offload registration code to the wrapper

The final list of OpenMP offload targets becomes known only at the link time and since offload registration code depends on the targets list it makes sense to delay offload registration code generation to the link time instead of adding it to the host part of every fat object. This patch moves offload registration code generation from clang to the offload wrapper tool.

This is the last part of the OpenMP linker script elimination patch https://reviews.llvm.org/D64943

Differential Revision: https://reviews.llvm.org/D68746

llvm-svn: 374937