Fix insufficient memory on some platforms

[tony2037]

[Problem]
The MEM_SIZE is hard-corded, but on some platforms, memory is insufficient.
Perhaps we should allow user to config accordingly

[Solution]
Expose an environment variable, CONFIG_MEM_SIZE, to allow user config the memory size with it

[Test]

• before $> make
  $> ./build/rv32emu ./build/hello.elf
  rv32emu: src/riscv.c:199: rv_create: Assertion `attr->mem' failed. Aborted (core dumped)

• after $> CONFIG_MEM_SIZE=0x40000000 make
  $> ./build/rv32emu ./build/hello.elf
  Hello World!
  Hello World!
  Hello World!
  Hello World!
  Hello World!
  inferior exit code 0

[Reference]
#488

[jserv]

I don't think the proposed change resolves the insufficient memory issue because it just relies on compile-time configurations instead of an adaptive approach to acquire the necessary system resources based on the requirements of RISC-V programs during instruction simulation.

@qwe661234, can you comment this?

[qwe661234]

This issue arises when the memory size is insufficient, especially when running rv32emu on a VM. However, arbitrarily setting a MEM_SIZE to pass specific tests can cause segmentation faults in other tests. A better solution is to revert to the original virtual memory implementation when memory size is insufficient, allocating memory only as needed for read or write operations.

[qwe661234]

After investigation, you can set MEM_SIZE to 0x80000 to run the benchmarks in directory tests. However, if you want to run some video games like doom or quake, the 4G memory is required.

[jserv]

Reworked in commit feca1de