[nvptx] Remove -m32

[nvptx] Remove -m32

The nvptx port has an -m32 option, but it's not clear whether this
was ever build/tested/used.

Don't expose to user anymore.  Tested on nvptx.

gcc/ChangeLog:

2020-10-15  Tom de Vries  <tdevries@suse.de>

	PR target/97436
	* config/nvptx/nvptx.opt (m32): Comment out.
	* doc/invoke.texi (NVPTX options): Remove -m32.

gcc/testsuite/ChangeLog:

2020-10-15  Tom de Vries  <tdevries@suse.de>

	PR target/97436
	* gcc.target/nvptx/atomic_fetch-3.c: Remove.