nvptx: Support '-mptx=5.0'

nvptx: Support '-mptx=5.0'

	gcc/
	* config/nvptx/nvptx-opts.h (enum ptx_version): Add
	'PTX_VERSION_5_0'.
	* config/nvptx/nvptx.cc (ptx_version_to_string)
	(ptx_version_to_number): Adjust.
	* config/nvptx/nvptx.h (TARGET_PTX_5_0): New.
	* config/nvptx/nvptx.opt (Enum(ptx_version)): Add 'EnumValue'
	'5.0' for 'PTX_VERSION_5_0'.
	* doc/invoke.texi (Nvidia PTX Options): Document '-mptx=5.0'.
	gcc/testsuite/
	* gcc.target/nvptx/mptx=5.0.c: New.

(cherry picked from commit 97616687149f115e0ab946b9a05a9f8c1e47429e)