Support musl libc
I've done something [here](https://github.com/gamecss/box64/tree/musl)
Seem able to compile on x86_64 and aarch64 alpine edge. Now I defining `box_malloc` as `malloc` in `src/include/debug.h`, but it stucks at first `box_malloc` now.
I'm a complete noob in C language..... I really need some suggestions :(
I have tried in defining something like `box_malloc` as `__libc_malloc`. With some hack, I'm able to link `__libc_*` without `__libc_realloc` then it stucks at `box_realloc'.