[Bug other/67092] bootstrap failure with CFLAGS/CXXFLAGS/BOOT_CFLAGS="-O0 -g" in running genpreds

[vries at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67092

--- Comment #1 from vries at gcc dot gnu.org ---
Created attachment 36106
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=36106&action=edit
tentative patch

This tentative patch:
- adds -Wl,-rpath settings to make sure genpreds can find the libstdc++ it
  linked against
- runs stage2+ genpreds with empty LD_LIBRARY_PATH, to ensure that the rpath
  is not overridden.

That allows the build to progress beyond failure point.