Improve memory/page init

Align heap start and caculate number of reserved pages
according to the length of ram available.

See
https://gitee.com/unicornx/riscv-operating-system-mooc/issues/I9LNCF.

Signed-off-by: Chen Wang <unicorn_wang@outlook.com>

code/os/07-hwtimer/platform.h

@@ -12,6 +12,9 @@
  */
 #define MAXNUM_CPU 8
 
+/* used in os.ld */
+#define LENGTH_RAM 128*1024*1024
+
 /*
  * MemoryMap
  * see https://github.com/qemu/qemu/blob/master/hw/riscv/virt.c, virt_memmap[]