[libomptarget][devicertl] Wrap source in declare target pragmas

[libomptarget][devicertl] Wrap source in declare target pragmas

Factored out of D93135 / D94745. C++ and cuda ignore unknown pragmas
so this is a NFC for the current implementation language. Removes noise
from patches for building deviceRTL as openmp.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D95048