[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.