[Runner] Push /opt/${target}/${target}/lib{,64} to linker search path

[giordano]

When compiling with GCC this makes sure its libraries have precedence over
anything else, in particular other compiler libraries that may be elsewhere, for
example CompilerSupportLibraries_jll used as dependency during the build.

Fix #163.

Note: I ran the tests of BinaryBuilder on this branch in JuliaPackaging/BinaryBuilder.jl#1032, seems all good.

[staticfloat]

Excellent quality as usual, Mosè.

[giordano]

Uhm, I suspect this can be a bit problematic when we want to link against libz, libxml2, or libiconv, because these libraries are also here 😞

sandbox:${WORKSPACE}/srcdir # ls /opt/${target}/${target}/lib64
libatomic.a                 libcilkrts.spec             libgomp.a                   libitm.a                    libobjc.so.4.0.0            libstdc++.so.6.0.23         libz.so.1
libatomic.so                libgcc_s.so                 libgomp.so                  libitm.so                   libquadmath.a               libstdc++.so.6.0.23-gdb.py  libz.so.1.2.11
libatomic.so.1              libgcc_s.so.1               libgomp.so.1                libitm.so.1                 libquadmath.so              libstdc++fs.a
libatomic.so.1.2.0          libgfortran.a               libgomp.so.1.0.0            libitm.so.1.0.0             libquadmath.so.0            libsupc++.a
libcilkrts.a                libgfortran.so              libgomp.spec                libitm.spec                 libquadmath.so.0.0.0        libxml2.so
libcilkrts.so               libgfortran.so.4            libiconv.so                 libobjc.a                   libstdc++.a                 libxml2.so.2
libcilkrts.so.5             libgfortran.so.4.0.0        libiconv.so.2               libobjc.so                  libstdc++.so                libxml2.so.2.9.12
libcilkrts.so.5.0.0         libgfortran.spec            libiconv.so.2.6.1           libobjc.so.4                libstdc++.so.6              libz.so