nvptx: In offloading compilation, special-case certain host-setup symbol aliases...

nvptx: In offloading compilation, special-case certain host-setup symbol aliases: avoid unused label 'emit_ptx_alias' diagnostic

Minor fix-up for commit 65b31b3fff2fced015ded1026733605f34053796
"nvptx: In offloading compilation, special-case certain host-setup symbol aliases [PR101544]",
as of which we see for non-offloading configurations:

    +[...]/source-gcc/gcc/config/nvptx/nvptx.cc: In function 'void nvptx_asm_output_def_from_decls(FILE*, tree, tree)':
    +[...]/source-gcc/gcc/config/nvptx/nvptx.cc:7769:2: warning: label 'emit_ptx_alias' defined but not used [-Wunused-label]
    + 7769 |  emit_ptx_alias:
    +      |  ^~~~~~~~~~~~~~

	gcc/
	* config/nvptx/nvptx.cc (nvptx_asm_output_def_from_decls)
	[!ACCEL_COMPILER]: Don't define label 'emit_ptx_alias'.