Unify brk()/sbrk()

[rennergade]

via: https://man7.org/linux/man-pages/man2/sbrk.2.html

   On Linux, sbrk() is implemented as a library function that uses
   the brk() system call, and does some internal bookkeeping so that
   it can return the old break value.

We currently implement brk and sbrk as separate syscalls though it seems like sbrk should be a wrapper. I'm not sure how trivial this is to refactor with our current implementation but it makes us use an additional syscall number that Linux doesn't have. If we could unify this in libc it would make our lives easier.