[gdb/contrib] Use OBJCOPY everywhere in cc-with-tweaks.sh

I noticed that the $want_gnu_debuglink code in gdb/contrib/cc-with-tweaks.sh
uses objcopy instead of $OBJCOPY.  Fix this.

Script checked with shellcheck, no new warnings added.

Tested on x86_64-linux.

gdb/contrib/cc-with-tweaks.sh

diff --git a/gdb/contrib/cc-with-tweaks.sh b/gdb/contrib/cc-with-tweaks.sh
index 7919d2d..c6f0f62 100755
--- a/gdb/contrib/cc-with-tweaks.sh
+++ b/gdb/contrib/cc-with-tweaks.sh
@@ -285,7 +285,7 @@
 
 	# Overwrite output_file with stripped version containing
 	# .gnu_debuglink to debug_file.
-	objcopy --add-gnu-debuglink="$link" "${stripped_file}" \
+	$OBJCOPY --add-gnu-debuglink="$link" "${stripped_file}" \
 		"${output_file}"
 	rc=$?
 	[ $rc != 0 ] && exit $rc