Flag to disable cling's `NVPTX` target

[LukasBreitwieser]

Explain what you would like to see improved and how.

In order to reduce binary size and compile time, @ktf asked for a way to disable the LLVM NVPTX build target.
Currently, NVPTX is added in all cases, even if ROOT is built with -Dcuda=off -Dcudnn=off:
see here:

if(NOT "${ROOT_CLING_TARGET}" STREQUAL "all")
  string(APPEND ROOT_CLING_TARGET ";NVPTX")
endif()

and a check here:

if( NOT "NVPTX" IN_LIST LLVM_TARGETS_TO_BUILD)
  message(FATAL_ERROR "NVPTX backend is not activated\n"
    "Please enable it via -DLLVM_TARGETS_TO_BUILD=\"host;NVPTX\"")
endif()

Commenting these lines leads to build errors related to the IncrementalCUDADeviceCompiler.

ROOT version

current master (e76402aad8)

Installation method

Build from sourc

Operating system

Ubuntu 24.04

Additional context

No response

[dpiparo]

This is a great opportunity to review whether we can remove even more targets, therewith reducing binary sizes and compile time.