prepare-cross-toolchain-unix: Avoid executing clang

Don't execute the source clang to check the resource directory;
when assembling a toolchain on a different system, we can't
assume that the source toolchain is executable. And when cross
building a toolchain for a foreign target, we can't assume we can
execute the target toolchain either.

prepare-cross-toolchain-unix.sh

@@ -25,7 +25,7 @@ DEST="$2"
 
 : ${ARCHS:=${TOOLCHAIN_ARCHS-i686 x86_64 armv7 aarch64}}
 
-CLANG_RESOURCE_DIR="$("$SRC/bin/clang" --print-resource-dir)"
+CLANG_RESOURCE_DIR="$(echo "$SRC/lib/clang/"*)"
 CLANG_VERSION=$(basename "$CLANG_RESOURCE_DIR")
 
 # Copy the clang resource files (include, lib, share). The clang cross