[compiler-rt][hwasan][fuchsia] Define shadow bound globals

These are required by MemIsShadow for checking if an address actually is shadow memory.

Differential Revision: https://reviews.llvm.org/D105745

compiler-rt/lib/hwasan/hwasan_fuchsia.cpp

@@ -34,6 +34,15 @@ bool InitShadow() {
   __sanitizer::InitShadowBounds();
   CHECK_NE(__sanitizer::ShadowBounds.shadow_limit, 0);
 
+  // These variables are used by MemIsShadow for asserting we have a correct
+  // shadow address. On Fuchsia, we only have one region of shadow, so the
+  // bounds of Low shadow can be zero while High shadow represents the true
+  // bounds. Note that these are inclusive ranges.
+  kLowShadowStart = 0;
+  kLowShadowEnd = 0;
+  kHighShadowStart = __sanitizer::ShadowBounds.shadow_base;
+  kHighShadowEnd = __sanitizer::ShadowBounds.shadow_limit - 1;
+
   return true;
 }