[libomptarget][devicertl] Drop templated atomic functions

[libomptarget][devicertl] Drop templated atomic functions

The five __kmpc_atomic templates are instantiated a total of seven times.
This change replaces the template with explictly typed functions, which
have the same prototype for amdgcn and nvptx, and implements them with
the same code presently in use.

Rolls in the accepted but not yet landed D95085.

The unsigned long long type can be replaced with uint64_t when replacing
the cuda function. Until then, clang warns on casting a pointer to one to
a pointer to the other.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D95093