[nvptx, testsuite] Fix gcc.target/nvptx/alias-*.c on sm_80

[nvptx, testsuite] Fix gcc.target/nvptx/alias-*.c on sm_80

When running test-cases gcc.target/nvptx/alias-*.c on target board
nvptx-none-run/-misa=sm_80 we run into fails because the test-cases add
-mptx=6.3, which doesn't support sm_80.

Fix this by only adding -mptx=6.3 if necessary, and simplify the test-cases by
using ptx_alias feature abstractions:
...
/* { dg-do run { target runtime_ptx_alias } } */
/* { dg-add-options ptx_alias } */
...

Tested on nvptx.

gcc/testsuite/ChangeLog:

2022-04-01  Tom de Vries  <tdevries@suse.de>

	* gcc.target/nvptx/nvptx.exp
	(check_effective_target_runtime_ptx_isa_version_6_3): Rename and
	generalize to ...
	(check_effective_target_runtime_ptx_isa_version_at_least): .. this.
	(check_effective_target_default_ptx_isa_version_at_least)
	(check_effective_target_runtime_ptx_alias, add_options_for_ptx_alias):
	New proc.
	* gcc.target/nvptx/alias-1.c: Use "target runtime_ptx_alias" and
	"dg-add-options ptx_alias".
	* gcc.target/nvptx/alias-2.c: Same.
	* gcc.target/nvptx/alias-3.c: Same.
	* gcc.target/nvptx/alias-4.c: Same.