use official API for unreachable C code (#3404)

diff --git a/src/_cffi_src/openssl/src/osrandom_engine.c b/src/_cffi_src/openssl/src/osrandom_engine.c
index 52f55af..f1cfcb6 100644
--- a/src/_cffi_src/openssl/src/osrandom_engine.c
+++ b/src/_cffi_src/openssl/src/osrandom_engine.c
@@ -335,7 +335,7 @@
         }
         return 1;
     }
-    return 0; /* unreachable */
+    __builtin_unreachable();
 }
 
 static int osrandom_finish(ENGINE *e) {
@@ -354,7 +354,7 @@
     case CRYPTOGRAPHY_OSRANDOM_GETRANDOM_WORKS:
         return 1;
     }
-    return 0; /* unreachable */
+    __builtin_unreachable();
 }
 
 static const char *osurandom_get_implementation(void) {
@@ -368,7 +368,7 @@
     case CRYPTOGRAPHY_OSRANDOM_GETRANDOM_WORKS:
         return "getrandom";
     }
-    return "<invalid>"; /* unreachable */
+    __builtin_unreachable();
 }
 #endif /* CRYPTOGRAPHY_OSRANDOM_ENGINE_GETRANDOM */