debuginfo lookup with sysroot case do not remove sysroot from file_name

debuginfo lookup with sysroot case do not remove sysroot from file_name

If sysroot option is passed, and debug symbols reside in sysroot along
with executable <foo> in <foo_dir>/.debug/<foo_file> directory,
stap fails to find debuginfo because it strips out sysroot path
from file_name so dwfl_standard_find_debuginfo ends up looking at
host <foo_dir>/.debug/<foo_file> rather then checking
<sysroot>/<foo_dir>/.debug/<foo_file>.