[nvptx] Add -mptx-comment

[nvptx] Add -mptx-comment

Add functionality that indicates which insns are added by -minit-regs, such
that for instance we have for pr53465.s:
...
        // #APP
// 9 "gcc/testsuite/gcc.c-torture/execute/pr53465.c" 1
        // Start: Added by -minit-regs=3:
        // #NO_APP
                mov.u32 %r26, 0;
        // #APP
// 9 "gcc/testsuite/gcc.c-torture/execute/pr53465.c" 1
        // End: Added by -minit-regs=3:
        // #NO_APP
...

Can be switched off using -mno-ptx-comment.

Tested on nvptx.

gcc/ChangeLog:

2022-02-21  Tom de Vries  <tdevries@suse.de>

	* config/nvptx/nvptx.cc (gen_comment): New function.
	(workaround_uninit_method_1, workaround_uninit_method_2)
	(workaround_uninit_method_3): : Use gen_comment.
	* config/nvptx/nvptx.opt (mptx-comment): New option.