stap-prep: switch to using main vmlinuz file as debuginfod test download

stap-prep: switch to using main vmlinuz file as debuginfod test download

Using the vdso* files only as the debuginfod tests gives us false
positives on platforms where the install vmlinuz files cannot be used
as a basis for debuginfod queries, because they're not elf nor
compressed-elf.  This new stap-prep tries to download the vmlinuz
debuginfo itself.  It's large, but at least once it's here, it's here!
And if it fails (as it will on those few platforms), the user is
advised to do full platform package-manager debuginfo download.