libroot/malloc: Increase address space reservation to 1GB.

Similar to the debug malloc and the old hoard2.

May help with #19592 a bit more.

src/system/libroot/posix/malloc/openbsd/PagesAllocator.cpp

@@ -27,7 +27,9 @@ static const size_t kPageSize = B_PAGE_SIZE;
 static const size_t kLargestUsefulChunk = 512 * kPageSize;
 
 /*! Amount of virtual address space to reserve when creating new areas. */
-static const size_t kReserveAddressSpace = 512 * 1024 * 1024;
+/* (The Haiku kernel will ignore reservations if there's no other address
+ * space left, so using a large value here should not hurt.) */
+static const size_t kReserveAddressSpace = 1 * 1024 * 1024 * 1024;
 
 /*! Cache up to this many percentage points of free memory (compared to used.) */
 static const size_t kFreePercentage = 25;