Fix the external GCC build after r297271 by setting -L <sysroot>/usr/lib.
GCC does add <sysroot>/usr/lib to the library search path but it comes after
/usr/local/lib which can find ports libraries such as libedit.so. The
bad path comes in as /usr/local/lib/gcc/x86_64-portbld-freebsd11.0/5.3.0/../../../
which corresponds to <prefix>/lib.
This partially reverts r297271.
Pointyhat to: bdrewery
Sponsored by: EMC / Isilon Storage Division