Replace use of internal syscalls

[nathanwhyte]

Closes #676

I found no internal uses of __sys_realloc() or __sys_free() anywhere.

I did find one reference to __sys_shutdown():
https://github.com/hermitcore/libhermit-rs/blob/df7483de1b762c570f5d52e48fc396cdb739a12d/src/arch/x86_64/kernel/processor.rs#L832-L836

It doesn't redirect to any functions in lib.rs like __sys_malloc() does. Does it need to be replaced as well?

[mkroening]

Thanks! This looks good to me 👍

bors r+

Regarding shutdown: I opened #718 to track that.
I also opened #717 for moving the sys_alloc functions as a follow-up.

[bors]

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