libroot_debug: Revert to a legacy default alignment of 8.

This reverts the legacy default alignment (in absence of max_align_t)
to 8, as it was before.

src/system/libroot/posix/malloc_debug/guarded_heap.cpp

@@ -38,7 +38,7 @@ static int32 sStackEndTLSIndex = -1;
 using namespace std;
 static size_t sDefaultAlignment = alignof(max_align_t);
 #else
-static size_t sDefaultAlignment = 0;
+static size_t sDefaultAlignment = 8;
 #endif
 
 

src/system/libroot/posix/malloc_debug/heap.cpp

@@ -54,7 +54,7 @@ static bool sUseGuardPage = false;
 using namespace std;
 static size_t sDefaultAlignment = alignof(max_align_t);
 #else
-static size_t sDefaultAlignment = 0;
+static size_t sDefaultAlignment = 8;
 #endif