sd-bus: set retain attribute on BUS_ERROR_MAP_ELF_REGISTER

LLD 13 and GNU ld 2.37 support -z start-stop-gc which allows garbage
collection of C identifier name sections despite the __start_/__stop_
references.  Simply set the retain attribute so that GCC 11 (if
configure-time binutils is 2.36 or newer)/Clang 13 will set the
SHF_GNU_RETAIN section attribute to prevent garbage collection.

Without the patch, there are linker errors like the following with -z
start-stop-gc.

```
ld: error: undefined symbol: __start_SYSTEMD_BUS_ERROR_MAP
>>> referenced by bus-error.c:93 (../src/libsystemd/sd-bus/bus-error.c:93)
>>>               sd-bus_bus-error.c.o:(bus_error_name_to_errno) in archive src/libsystemd/libsystemd_static.a
```

src/libsystemd/sd-bus/bus-error.h

@@ -28,11 +28,17 @@ int bus_error_set_errnofv(sd_bus_error *e, int error, const char *format, va_lis
  * the bus error table, and BUS_ERROR_MAP_ELF_USE has to be used at
  * least once per compilation unit (i.e. per library), to ensure that
  * the error map is really added to the final binary.
+ *
+ * In addition, set the retain attribute so that the section cannot be
+ * discarded by ld --gc-sections -z start-stop-gc. Older compilers would
+ * warn for the unknown attribute, so just disable -Wattributes.
  */
 
 #define BUS_ERROR_MAP_ELF_REGISTER                                      \
+        _Pragma("GCC diagnostic ignored \"-Wattributes\"")              \
         _section_("SYSTEMD_BUS_ERROR_MAP")                              \
         _used_                                                          \
+        __attribute__((retain))                                         \
         _alignptr_                                                      \
         _variable_no_sanitize_address_