[libomptarget] Remove duplicate RTLRequiresFlags per device

We have one global RTLs.RequiresFlags, I don't see a need to make a
copy per device that the runtime manages. This was problematic anyway
because the copy happened during the first __tgt_register_lib(). This
made it impossible to call __tgt_register_requires() from normal user
funtions for testing.
Hence, this change also fixes unified_shared_memory/shared_update.c for
older versions of Clang that don't call __tgt_register_requires() before
__tgt_register_lib().

Differential Revision: https://reviews.llvm.org/D66019

llvm-svn: 368465