Ports: Set $LD for Clang, GCC and host in .hosted_defs.sh

Sets `$LD` to `$HOST_LD` in `.port_include.sh` if it is defined as well,
else it will be set to `ld`.

Makes libiconv build with the Clang toolchain.

This also impacts other ports when building them with the Clang
toolchain, and might result in more ports building correctly.

Ports/.port_include.sh

@@ -64,6 +64,7 @@ enable_ccache
 host_env() {
     export CC="${HOST_CC}"
     export CXX="${HOST_CXX}"
+    export LD="${HOST_LD}"
     export AR="${HOST_AR}"
     export RANLIB="${HOST_RANLIB}"
     export PATH="${HOST_PATH}"