In preparation of GCC/nvptx code changes that require sm_52 features, this
commit raises nvptx code generation from sm_30 "Kepler" to sm_52 "Maxwell".
The latter has been supported as of CUDA 6.5 (2014-08), and is thus supported
by most Nvidia GPUs of the last decade, approximately.  (This commit doesn't
change the use of PTX ISA 6.0, which already requires CUDA 9.0 anyway.)

To continue building sm_30 multilib variants (for use via building/linking with
'-march=sm_30'), specify '--with-multilib-list=default,sm_30', for example.  Or,
to continue defaulting to sm_30 multilib variants, specify '--with-arch=sm_30'
(plus '--without-multilib-list', if applicable).  See the documentation,
<https://gcc.gnu.org/install/specific.html#nvptx-x-none>.

(Note that after a long deprecation time, eventually the
sm_3x "Kepler architecture support is removed from CUDA 12.0", 2022-12.)

	gcc/
	* config.gcc [nvptx-*]: Switch default from '-march=sm_30' to
	'-march=sm_52'.
	* doc/install.texi (Nvidia PTX Options): Update.