hurd: Fix unwinding over INTR_MSG_TRAP in shared too

This follows 63550530d9 ("hurd: Fix unwinding over INTR_MSG_TRAP"),
for the shared library case.

hurd/Makefile

@@ -92,6 +92,7 @@ CFLAGS-RPC_exec_startup_get_info.o = $(no-stack-protector)
 
 # Make it simpler to unwind over INTR_MSG_TRAP
 CFLAGS-intr-msg.o = -fno-omit-frame-pointer
+CFLAGS-intr-msg.os = -fno-omit-frame-pointer
 
 # Make generated headers compatible with all support standards
 migheaderpipe := | sed -e 's/\<ino64_t\>/__ino64_t/' -e 's/\<loff_t\>/__loff_t/'