Mark vDSO as not a file

Mark vDSO as not a file

The vDSO objfile is not a real file, so mark it as such.  I noticed
this because, when playing with debuginfod, I saw:

Downloading 0.01 MB separate debug info for /tmp/system-supplied DSO at 0x7ffff7fc9000

That "/tmp" is wrong -- it's just gdb's cwd.  This patch corrects the
problem, resulting in:

Downloading 0.01 MB separate debug info for system-supplied DSO at 0x7ffff7fc9000

Regression tested on x86-64 Fedora 34.