[nvptx] Update help text for m64
In the docs we have for m64:
...
Ignored, but preserved for backward compatibility. Only 64-bit ABI is
supported.
...
But with --target-help, we have instead:
...
$ gcc --target-help
...
-m64 Generate code for a 64-bit ABI.
...
which could be interpreted as meaning that generating code for a 32-bit ABI is
still possible.
Fix this by instead emitting the same text as in the docs:
...
-m64 Ignored, but preserved for backward compatibility. Only 64-bit
ABI is supported.
...
Tested on nvptx.
gcc/ChangeLog:
2022-03-29 Tom de Vries <tdevries@suse.de>
* config/nvptx/nvptx.opt (m64): Update help text to reflect that it
is ignored.