[nvptx] Fix dummy location in gen_comment

[nvptx] Fix dummy location in gen_comment

I committed "[nvptx] Add -mptx-comment", but tested it in combination with the
proposed "[final] Handle compiler-generated asm insn" (
https://gcc.gnu.org/pipermail/gcc-patches/2022-February/590721.html ), so
by itself the commit introduced some regressions:
...
FAIL: gcc.dg/20020426-2.c (internal compiler error: Segmentation fault)
FAIL: gcc.dg/analyzer/zlib-3.c (internal compiler error: Segmentation fault)
FAIL: gcc.dg/pr101223.c (internal compiler error: Segmentation fault)
FAIL: gcc.dg/torture/pr80764.c   -O2  (internal compiler error: Segmentation fault)
...

There are due to cfun->function_start_locus == 0.

Fix these by using DECL_SOURCE_LOCATION (cfun->decl) instead.

Tested on nvptx.

gcc/ChangeLog:

2022-02-23  Tom de Vries  <tdevries@suse.de>

	* config/nvptx/nvptx.cc (gen_comment): Use
	DECL_SOURCE_LOCATION (cfun->decl) instead of cfun->function_start_locus.