Move from 'gcc.target/nvptx/nvptx.exp' into 'target-supports.exp' additions for nvptx...

Move from 'gcc.target/nvptx/nvptx.exp' into 'target-supports.exp' additions for nvptx target

	gcc/testsuite/
	* gcc.target/nvptx/nvptx.exp
	(check_effective_target_default_ptx_isa_version_at_least)
	(check_effective_target_default_ptx_isa_version_at_least_6_0)
	(check_effective_target_runtime_ptx_isa_version_at_least)
	(check_effective_target_runtime_ptx_alias)
	(add_options_for_ptx_alias): Move...
	* lib/target-supports.exp
	(check_nvptx_default_ptx_isa_version_at_least)
	(check_effective_target_nvptx_default_ptx_isa_version_at_least_6_0)
	(check_nvptx_runtime_ptx_isa_version_at_least)
	(check_effective_target_nvptx_runtime_alias_ptx)
	(add_options_for_nvptx_alias_ptx): ... here.
	* gcc.target/nvptx/alias-1.c: Adjust.
	* gcc.target/nvptx/alias-2.c: Likewise.
	* gcc.target/nvptx/alias-3.c: Likewise.
	* gcc.target/nvptx/alias-4.c: Likewise.
	* gcc.target/nvptx/alias-to-alias-1.c: Likewise.
	* gcc.target/nvptx/alias-weak-1.c: Likewise.
	* gcc.target/nvptx/uniform-simt-5.c: Likewise.
	gcc/
	* doc/sourcebuild.texi (Effective-Target Keywords): Document
	'nvptx_default_ptx_isa_version_at_least_6_0',
	'nvptx_runtime_alias_ptx'.
	(Add Options): Document 'nvptx_alias_ptx'.