libgomp.texi: For HIP interop, mention cpp defines to set

libgomp.texi: For HIP interop, mention cpp defines to set

The HIP header files recognize the used compiler, defaulting to either AMD
or Nvidia/CUDA; thus, the alternative way of explicitly defining a macro is
less prominently documented. With GCC, the user has to define the
preprocessor macro manually. Hence, as a service to the user, mention
__HIP_PLATFORM_AMD__ and __HIP_PLATFORM_NVIDIA__ in the interop documentation,
even though it has only indirectly to do with GCC and its interop support.

Note to commit-log readers, only: For Fortran, the hipfort modules can be
used; when compiling the hipfort package (defaults to use gfortran), it
generates the module (*.mod) files in include/hipfort/{amdgcn,nvidia}/ such
that the choice is made by setting the respective include path.

libgomp/ChangeLog:

	* libgomp.texi (gcn interop, nvptx interop): For HIP with C/C++, add
	a note about setting a preprocessor define.