Added "nop" to IM_DEBUG_BREAK macro on GCC to work around GDB bug (#7266)

There are two issues here - first, this macro uses AT&T specific syntax with $, which is not necessary. Also, some assemblers (nasm) emit different bytes for "int 3" and "int3", so it's better to use "int3" (cd 03 vs cc)

More importantly, GDB has some failing assertion whenever stepping after hitting an "int3" instruction. This makes it practically useless, as is. For some reason, putting a nop afterwards as a workaround is okay.

Related discussions:
https://sourceware.org/bugzilla/show_bug.cgi?id=31194
https://lists.sr.ht/~skeeto/public-inbox/%3C2d3d7662a361ddd049f7dc65b94cecdd%40disroot.org%3E

docs/CHANGELOG.txt

@@ -56,6 +56,7 @@ Other changes:
 - Nav: Fixed pressing Escape while in a child window with _NavFlattened flag. (#7237)
 - Debug Tools: Metrics: Fixed debug break in SetShortcutRouting() not handling ImGuiMod_Shortcut redirect.
 - Debug Tools: Debug Log: Added "Input Routing" logging.
+- Debug Tools: Added "nop" to IM_DEBUG_BREAK macro on GCC to work around GDB bug (#7266) [@Peter0x44]
 - Backends: Vulkan: Fixed vkAcquireNextImageKHR() validation errors in VulkanSDK 1.3.275 by
   allocating one extra semaphore than in-flight frames. (#7236) [@mklefrancois]
 - Backends: Vulkan: Fixed vkMapMemory() calls unnecessarily using full buffer size. (#3957)

imgui_internal.h

@@ -300,11 +300,11 @@ namespace ImStb
 #elif defined(__clang__)
 #define IM_DEBUG_BREAK()    __builtin_debugtrap()
 #elif defined(__GNUC__) && (defined(__i386__) || defined(__x86_64__))
-#define IM_DEBUG_BREAK()    __asm__ volatile("int $0x03")
+#define IM_DEBUG_BREAK()    __asm__ volatile("int3;nop")
 #elif defined(__GNUC__) && defined(__thumb__)
 #define IM_DEBUG_BREAK()    __asm__ volatile(".inst 0xde01")
 #elif defined(__GNUC__) && defined(__arm__) && !defined(__thumb__)
-#define IM_DEBUG_BREAK()    __asm__ volatile(".inst 0xe7f001f0");
+#define IM_DEBUG_BREAK()    __asm__ volatile(".inst 0xe7f001f0")
 #else
 #define IM_DEBUG_BREAK()    IM_ASSERT(0)    // It is expected that you define IM_DEBUG_BREAK() into something that will break nicely in a debugger!
 #endif