[PTX] rework PTX prototype emission

[Nathan Sidwell]

This patch moves all PTX prototype emission from a DECL into the renamed 
'write_fn_decl'.  Thus we no longer need write_fn_decl_and_comment, nor does 
nvptx_declare_function_name need to emit the linker comment marker itself (which 
is nearly identical to the one that was emitted by write_fn_decl_and_comment).

While there I made the handling of name replacement consistent between the two 
prototype emitters and tidied up the argument emission of write_fn_decl.

nathan
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