gcc: fix building darwin->linux cross compiler

This reverts d9feea5 with some slight modifications to work with
other changes since then.

Fixes #88213.

pkgs/development/compilers/gcc/builder.sh

@@ -240,7 +240,7 @@ postInstall() {
     # More dependencies with the previous gcc or some libs (gccbug stores the build command line)
     rm -rf $out/bin/gccbug
 
-    if type "patchelf"; then
+    if [[ buildConfig == *"linux"* ]]; then
         # Take out the bootstrap-tools from the rpath, as it's not needed at all having $out
         for i in $(find "$out"/libexec/gcc/*/*/* -type f -a \! -name '*.la'); do
             PREV_RPATH=`patchelf --print-rpath "$i"`
@@ -249,6 +249,16 @@ postInstall() {
         done
     fi
 
+    if [[ targetConfig == *"linux"* ]]; then
+        # For some reason, when building for linux on darwin, the libs retain
+	# RPATH to $out.
+        for i in "$lib"/"$targetConfig"/lib/{libtsan,libasan,libubsan}.so.*.*.*; do
+            PREV_RPATH=`patchelf --print-rpath "$i"`
+            NEW_RPATH=`echo "$PREV_RPATH" | sed "s,:${out}[^:]*,,g"`
+            patchelf --set-rpath "$NEW_RPATH" "$i" && echo OK
+        done
+    fi
+
     if type "install_name_tool"; then
         for i in "$lib"/lib/*.*.dylib; do
             install_name_tool -id "$i" "$i" || true