[OpenMP] Unconditionally link the OpenMP device RTL static library

Currently we have two versions of the static library. One is built as
individual bitcode files and linked via `-mlink-builtin-bitcode`. The
other is built as a single static archive `omptarget.devicertl.a` and is
linked via `-lomptarget.devicertl` and handled by the linker wrapper
during LTO. We use the former in the case that we are not performing
LTO, because linking the library late wouldn't allow us to optimize the
runtime library effectively. The support in D142484 allows us to
unconditionally link this library, so it will only be pulled in if
needed. That is, if we linked already via `-mlink-builtin-bitcode` then
we will not pull in the static library even if it's linked on the
command line.

Depends on D142484

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D142486