Set debug-file-directory when using a sysroot

debug-file-directory is the directory where gdb goes to look for
detached debug symbols (the non buildid kind). It's interaction with the
sysroot bits are bit.. odd.. For each debug-file-directory path it will
look in ${path}/${sysroot}/<debuglink> and ${path}/debuglink, both of
which are mostly useless  for sysroots.

Explicitely set the path to ${sysroot}/usr/lib/debug to set gdb
straight.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Reviewed-by: Guillaume Desmottes <guillaume.desmottes@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D5304