glibc: Enable separate debug symbols

The importance of glibc makes it worthwhile to provide debug
symbols. However, this revealed an issue with separateDebugInfo: it
was indiscriminately adding --build-id to all ld invocations, while in
fact it should only do that for final links. Glibc also uses non-final
("relocatable") links, leading to subsequent failure to apply a build
ID ("Cannot create .note.gnu.build-id section, --build-id
ignored"). So now ld-wrapper.sh only passes --build-id for final
links.

pkgs/build-support/cc-wrapper/ld-wrapper.sh

@@ -146,11 +146,26 @@ if [ "$NIX_DONT_SET_RPATH" != 1 ]; then
 
     # Finally, add `-rpath' switches.
     for i in $rpath; do
-        extra=(${extra[@]} -rpath $i)
+        extra+=(-rpath $i)
     done
 fi
 
 
+# Only add --build-id if this is a final link. FIXME: should build gcc
+# with --enable-linker-build-id instead?
+if [ "$NIX_SET_BUILD_ID" = 1 ]; then
+    for p in "${params[@]}"; do
+        if [ "$p" = "-r" -o "$p" = "--relocatable" -o "$p" = "-i" ]; then
+            relocatable=1
+            break
+        fi
+    done
+    if [ -z "$relocatable" ]; then
+        extra+=(--build-id)
+    fi
+fi
+
+
 # Optionally print debug info.
 if [ -n "$NIX_DEBUG" ]; then
   echo "original flags to @prog@:" >&2