[OpenMP][NVPTX] Take functions in `deviceRTLs` as `convergent`

OpenMP device compiler (similar to other SPMD compilers) assumes that
functions are convergent by default to avoid invalid transformations, such as
the bug (https://bugs.llvm.org/show_bug.cgi?id=49021).

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D95971

GitOrigin-RevId: 0f0ce3c12edefd25448e39c4d20718a10d3d42c1