gcc: fix cross-compiling by helping some configure tests to find the libc for linking

Note:

While trying to cross-compile a uclibc based target I experienced failing
configure tests for computing the suffix of object files.

In the respective config.log following error occured multiple times:
TOOLCHAIN/tools.cross/crosscc/../libexec/gcc/i686-unknown-linux-uclibc/4.4.3/cc1: error while loading shared libraries: libc.so.0: cannot open shared object file: No such file or directory

No matter if that problem occured only while building a uclibc target, it seems
to be a good idea to assure that we link against the right libc.

base/gcc/gcc.conf

@@ -138,6 +138,8 @@ if atstage cross; then
 	# for gcc versions 4.3 and upwards we have to explicitely specifiy the
 	# target architecture at cross-compiling stage
 	var_append confopt ' ' '--target=$arch_target'
+	# some configure tests of gcc > 4.3 need some help for linking
+	var_append GCC_WRAPPER_APPEND ' ' "-Wl,-rpath,${root}/lib"
 fi
 
 if ! atstage native; then