NVHPC: Add support for CMAKE_<LANG>_COMPILER_EXTERNAL_TOOLCHAIN

NVHPC has a `--gcc-toolchain=` flag just like LLVM/Clang and IntelLLVM.

Modules/Compiler/NVHPC.cmake

@@ -13,5 +13,6 @@ include(Compiler/PGI)
 macro(__compiler_nvhpc lang)
   # Logic specific to NVHPC.
   set(CMAKE_INCLUDE_SYSTEM_FLAG_${lang} "-isystem ")
+  set(CMAKE_${lang}_COMPILE_OPTIONS_EXTERNAL_TOOLCHAIN "--gcc-toolchain=")
   set(CMAKE_${lang}_COMPILE_OPTIONS_WARNING_AS_ERROR "-Werror")
 endmacro()