[Libomptarget] Add `cold` to KeepAlive attributes

This patch adds the `cold` attribute to the keepAlive functions in the
RTL. This dummy function exists to keep certain RTL calls alive without
them being optimized out, but it is never called and can be declared
cold. This also helps some erroneous remarks being given on this
function because it has weak linkage and cannot be made internal.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D117513