We're setting -fdebug-prefix-map to strip directory prefixes from debug
info, which will help ccache and reproducibility.
However, -fdebug-prefix-map doesn't help with those macros like __FILE__
and __BASE_FILE__, which needs another flags: -fmacro-prefix-map.
Replaces -fdebug-prefix-map with -ffile-prefix-map which is an alias for
both `-fdebug-prefix-map` and `-fmacro-prefix-map`. (This flag is
available since GCC 8 and Clang 10)
3dcac23aff