gcc50: Switch final specs to set DT_RUNPATH tag

gcc50: Switch final specs to set DT_RUNPATH tag

The system dynamic binaries have always been built with DT_RPATH set
by the linker.  A few years ago the dynamic linker gained DT_RUNPATH
capability but it hasn't been used in base.

How it works is that when the linker is given the -rpath switch, it
will always set DT_RPATH, but if the --enable-new-dtags switch is present,
the linker will also set DT_RUNPATH tag.  The dynamic linker reacts
differently when DT_RUNPATH is present; it will check LD_LIBRARY_PATH
before the rpath in that case.

This change (to gcc50 only) builds the system with the -rpath flag alone
by for the ctools compiler, but the final compiler's specs include the
--enable-new-dtags switch as well, which affects the building of ports and
all non-system use of the compiler.