[HIP] fix host-used external kernel (#83870)

In -fgpu-rdc mode, when an external kernel is used by a host function
with weak_odr linkage (e.g. explicitly instantiated template function),
the kernel should not be marked as host-used external kernel, since the
host function may be dropped by the linker. Mark the external kernel as
host-used external kernel will force a reference to the external kernel,
which the user may not define in other TU.

Fixes: https://github.com/llvm/llvm-project/issues/83771