[llvm][AddressSanitizer][BPF] add default shadow mapping offset for BPF target (#167768)

The AddressSanitizer transform does not have a default offset registered
for the shadow map. Set the default shadow map offset for BPF be
dynamically set by the KASAN implementation.

Author: etsal

llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp

@@ -508,6 +508,7 @@ static ShadowMapping getShadowMapping(const Triple &TargetTriple, int LongSize,
   bool IsAMDGPU = TargetTriple.isAMDGPU();
   bool IsHaiku = TargetTriple.isOSHaiku();
   bool IsWasm = TargetTriple.isWasm();
+  bool IsBPF = TargetTriple.isBPF();
 
   ShadowMapping Mapping;
 
@@ -584,6 +585,8 @@ static ShadowMapping getShadowMapping(const Triple &TargetTriple, int LongSize,
     else if (IsHaiku && IsX86_64)
       Mapping.Offset = (kSmallX86_64ShadowOffsetBase &
                         (kSmallX86_64ShadowOffsetAlignMask << Mapping.Scale));
+    else if (IsBPF)
+      Mapping.Offset = kDynamicShadowSentinel;
     else
       Mapping.Offset = kDefaultShadowOffset64;
   }