nvptx: Add a __PTX_SM__ predefined macro based on target ISA.

nvptx: Add a __PTX_SM__ predefined macro based on target ISA.

This patch adds a __PTX_SM__ predefined macro to the nvptx backend that
allows code to check the compute model being targeted by the compiler.
This is equivalent to the __CUDA_ARCH__ macro defined by CUDA's nvcc
compiler, but to avoid causing problems for source code that checks
for that compiler, this macro uses GCC's nomenclature; it's easy
enough for users to "#define __CUDA_ARCH__ __PTX_SM__".

What might have been a four line patch is actually a little more
complicated, as this patch takes the opportunity to upgrade the
nvptx backend to use the now preferred nvptx-c.c idiom.

2021-08-24  Roger Sayle  <roger@nextmovesoftware.com>
	    Tom de Vries  <tdevries@suse.de>

gcc/ChangeLog
	* config.gcc (nvptx-*-*): Define {c,c++}_target_objs.
	* config/nvptx/nvptx-protos.h (nvptx_cpu_cpp_builtins): Prototype.
	* config/nvptx/nvptx.h (TARGET_CPU_CPP_BUILTINS): Implement with
	a call to the new nvptx_cpu_cpp_builtins function in nvptx-c.c.
	* config/nvptx/t-nvptx (nvptx-c.o): New rule.
	* config/nvptx/nvptx-c.c: New source file.
	(nvptx_cpu_cpp_builtins): Move implementation here.