nvptx: Expose '-mptx=4.2'

nvptx: Expose '-mptx=4.2'

'PTX_VERSION_4_2' was added in commit decde11183bdccc46587d6614b75f3d56a2f2e4a
"[nvptx] Choose -mptx default based on -misa" for use for '-march=sm_52'
('first_ptx_version_supporting_sm', 'PTX_ISA_SM53'), as documented by Nvidia.
However, '-mptx=4.2' wasn't exposed to the user, but there's no reason not to.

	gcc/
	* config/nvptx/nvptx.h (TARGET_PTX_4_2): New.
	* config/nvptx/nvptx.opt (Enum(ptx_version)): Add 'EnumValue'
	'4.2' for 'PTX_VERSION_4_2'.
	* doc/invoke.texi (Nvidia PTX Options): Document '-mptx=4.2'.
	gcc/testsuite/
	* gcc.target/nvptx/mptx=4.2.c: New.