[Bug debug/68848] allow -fdebug-prefix-map to read OLD prefix from environment (improve reproducibility)

[dkg at fifthhorseman dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68848

--- Comment #5 from Daniel Kahn Gillmor <dkg at fifthhorseman dot net> ---
Created attachment 37007
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=37007&action=edit
ignore -fdebug-prefix-map when generating DW_AT_producer

Here is an alternate approach (suggested by Bernd Schmidt): just avoid writing
-fdebug-prefix-map to the DW_AT_producer field entirely.