gcc pass2: use --target=$LFS_TGT instead of many {C,CXX}_FOR_TARGET

There is also a "GCC_FOR_TARGET" which was not set correctly.  The
--target=$LFS_TGT parameter can set up all of these variables as we
expected.

chapter06/gcc-pass2.xml

@@ -88,10 +88,9 @@ cd       build</userinput></screen>
 <screen><userinput remap="configure">../configure                                       \
     --build=$(../config.guess)                     \
     --host=$LFS_TGT                                \
-    --prefix=/usr                                  \
-    CC_FOR_TARGET=$LFS_TGT-gcc                     \
-    CXX_FOR_TARGET=$LFS_TGT-g++                    \
+    --target=$LFS_TGT                              \
     LDFLAGS_FOR_TARGET=-L$PWD/$LFS_TGT/libgcc      \
+    --prefix=/usr                                  \
     --with-build-sysroot=$LFS                      \
     --enable-initfini-array                        \
     --disable-nls                                  \