DWARF prototypes are now generated with both the plain and the linkage names

If a die has a DW_AT_linkage_name, I now use it: this is required for C++ code,
in particular.

I use the plain name regardless, since sometimes the exported symbol corresponds
to the plain name, NOT the linkage name. For instance I see this on my
Debian/sid amd64 box. In its libc, the linkage name of __nanosleep is
__GI___nanosleep, but the export is __nanosleep