Fix the external GCC build after r297271 by setting -L <sysroot>/usr/lib.

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