[asan] Fix small X86_64 ShadowOffset for non-default shadow scale
The requirement is that shadow memory must be aligned to page
boundaries (4k in this case).  Use a closed form equation that always
satisfies this requirement.
@@ -139,7 +139,8 @@ static const u64 kDefaultShadowScale = 3;
static const u64 kDefaultShadowSentinel = ~(uptr)0;
static const u64 kDefaultShadowOffset32 = 1ULL << 29; // 0x20000000
static const u64 kDefaultShadowOffset64 = 1ULL << 44;
-static const u64 kDefaultShort64bitShadowOffset = 0x7FFF8000; // < 2G.
+static const u64 kDefaultShort64bitShadowOffset =
+ 0x7FFFFFFF & (~0xFFFULL << kDefaultShadowScale); // < 2G.
static const u64 kIosShadowOffset32 = 1ULL << 30; // 0x40000000
static const u64 kIosShadowOffset64 = 0x120200000;
static const u64 kIosSimShadowOffset32 = 1ULL << 30;
