[CUDA][HIP] Externalize kernels with internal linkage

This patch is a continuation of https://reviews.llvm.org/D123353.

Not only kernels in anonymous namespace, but also template
kernels with template arguments in anonymous namespace
need to be externalized.

To be more generic, this patch checks the linkage of a kernel
assuming the kernel does not have __global__ attribute. If
the linkage is internal then clang will externalize it.

This patch also fixes the postfix for externalized symbol
since nvptx does not allow '.' in symbol name.

Reviewed by: Artem Belevich

Differential Revision: https://reviews.llvm.org/D124189

Fixes: https://github.com/llvm/llvm-project/issues/54560