fix typo

Thanks to "Gabor Greif" <ggreif@gmail.com> for reporting this problem.

The configure flag should be --with-default-sysroot as documented, and
not --with-sysroot.  The reason we don't want to define --with-sysroot
is that GCC has a configure flag by that name and it has a different
semantics.

llvm-svn: 155844