Use __builtin_trap() on GCC too.

Source/wtf/Assertions.h

 /*
  * Copyright (C) 2003, 2006, 2007 Apple Inc.  All rights reserved.
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 118 matching lines @@
 void WTFSetCrashHook(WTFCrashHookFunction);
 void WTFInvokeCrashHook();
 void WTFInstallReportBacktraceOnCrashHook();
 
 #ifdef __cplusplus
 }
 #endif
 
 /* IMMEDIATE_CRASH() - Like CRASH() below but crashes in the fastest, simplest possible way with no attempt at logging. */
 #ifndef IMMEDIATE_CRASH
-#if COMPILER(CLANG)
+#if COMPILER(GCC)
 #define IMMEDIATE_CRASH() __builtin_trap()
 #else
 #define IMMEDIATE_CRASH() ((void(*)())0)()
 #endif
 #endif
 
 /* CRASH() - Raises a fatal error resulting in program termination and triggering either the debugger or the crash reporter.
 
    Use CRASH() in response to known, unrecoverable errors like out-of-memory.
    Macro is enabled in both debug and release mode.
@@ skipping 217 matching lines @@
 #define RELEASE_ASSERT(assertion) (UNLIKELY(!(assertion)) ? (IMMEDIATE_CRASH()) : (void)0)
 #define RELEASE_ASSERT_WITH_MESSAGE(assertion, ...) RELEASE_ASSERT(assertion)
 #define RELEASE_ASSERT_NOT_REACHED() IMMEDIATE_CRASH()
 #else
 #define RELEASE_ASSERT(assertion) ASSERT(assertion)
 #define RELEASE_ASSERT_WITH_MESSAGE(assertion, ...) ASSERT_WITH_MESSAGE(assertion, __VA_ARGS__)
 #define RELEASE_ASSERT_NOT_REACHED() ASSERT_NOT_REACHED()
 #endif
 
 #endif /* WTF_Assertions_h */