syscall: Zero out memory that already exists during the brk system call.

syscall: Zero out memory that already exists during the brk system call.

Glibc often assumes that memory it receives from the kernel after a brk
system call will contain only zeros. This is important during a calloc,
because it won't clear the new memory itself. In the simulator, if the
new page exists, it will be cleared using this patch, to mimic the kernel's
functionality.