boot: Fix undefined reference to raise() on arm

This is just a workaround. Once we drop gnu-efi, the arm build system
for EFI binaries should be changed to use the arm-none-eabi toolchain,
which should not exhibit this behavior.

src/boot/efi/util.c

@@ -721,3 +721,9 @@ void *find_configuration_table(const EFI_GUID *guid) {
 
         return NULL;
 }
+
+/* libgcc's __aeabi_ldiv0 intrinsic will call raise() on division by zero, so we
+ * need to provide one ourselves for now. */
+_noreturn_ int raise(int sig) {
+        assert_not_reached();
+}