Allocate adequate memory for running arch-test

Once the memory mapping requirement is adjusted to 2^{19} - 1 bytes,
RISC-V Architecture Tests is unable to run:

  make[1]: *** [rv32emu/build/arch-test/Makefile.DUT-rv32emu:5: TARGET0] Error 139
  INFO | Running Tests on Reference Model.
  INFO | Initiating signature checking.
  ERROR | Signature file : arch-test/rv32i_m/A/src/amoadd.w-01.S/dut/DUT-rv32emu.signature does not exist

Close #484

src/main.c

@@ -174,8 +174,8 @@ static void dump_test_signature(const char *prog_name)
 #endif
 /* FIXME: MEM_SIZE shall be defined on different runtime */
 #ifndef MEM_SIZE
-/* Allocate 2^{19} - 1 bytes, which is ample for all known benchmarks. */
-#define MEM_SIZE 0x80000
+/* Allocate 2^{19} bytes, which is ample for all known benchmarks. */
+#define MEM_SIZE 0x80000ULL
 #endif
 #define STACK_SIZE 0x1000       /* 4096 */
 #define ARGS_OFFSET_SIZE 0x1000 /* 4096 */