[asan] Add a magic shadow value for shadw gap

[waltl]

Summary

This gives us something to insert into the shadow gap for systems that
don't have memory protection turned on there (i.e. on Myriad).

[kcc]

compiler-rt/lib/asan/asan_errors.cc

517

Maybe "Shadow gap" and kAsanShadowGap?

compiler-rt/lib/asan/asan_internal.h

150

add a comment why you need it.

[waltl]

Address CR comments

compiler-rt/lib/asan/asan_errors.cc

PrintShadowByte(str, "  Container overflow:      ",
                   kAsanContiguousContainerOOBMagic);
   PrintShadowByte(str, "  Array cookie:            ",
                   kAsanArrayCookieMagic);
   PrintShadowByte(str, "  Intra object redzone:    ",
                   kAsanIntraObjectRedzone);
   PrintShadowByte(str, "  ASan internal:           ", kAsanInternalHeapMagic);
   PrintShadowByte(str, "  Left alloca redzone:     ", kAsanAllocaLeftMagic);
   PrintShadowByte(str, "  Right alloca redzone:    ", kAsanAllocaRightMagic);
+  PrintShadowByte(str, "  Shadow gap:              ", kAsanShadowGap);
 }
 
 static void PrintShadowBytes(InternalScopedString *str, const char *before,
                              u8 *bytes, u8 *guilty, uptr n) {
   Decorator d;
   if (before) str->append("%s%p:", before, bytes);
   for (uptr i = 0; i < n; i++) {
     u8 *p = bytes + i;

compiler-rt/lib/asan/asan_internal.h

 const int kAsanContiguousContainerOOBMagic = 0xfc;
 const int kAsanStackUseAfterScopeMagic = 0xf8;
 const int kAsanGlobalRedzoneMagic = 0xf9;
 const int kAsanInternalHeapMagic = 0xfe;
 const int kAsanArrayCookieMagic = 0xac;
 const int kAsanIntraObjectRedzone = 0xbb;
 const int kAsanAllocaLeftMagic = 0xca;
 const int kAsanAllocaRightMagic = 0xcb;
+// Used to populate the shadow gap for systems without memory
+// protection there (i.e. Myriad).
+const int kAsanShadowGap = 0xcc;
 
 static const uptr kCurrentStackFrameMagic = 0x41B58AB3;
 static const uptr kRetiredStackFrameMagic = 0x45E0360E;
 
 }  // namespace __asan
 
 #endif  // ASAN_INTERNAL_H