Use svcGetInfo to get the address space in find_free_address

[todo]

We should use svcGetInfo to get the address space in `find_free_address`. This is extremely important as the low addresses (from 0 to ADDRESS_SPACE_MIN) are not usable, but are marked as available by QueryMemory.
Here's a sample implementation for when we'll have GetInfo impl'd.
```rust lazy_static! { static ref ADDRESS_SPACE: (usize, usize) = { let addr_space_base = syscalls::get_info(Process::current(), 12, 0).unwrap(); let addr_space_size = syscalls::get_info(Process::current(), 13, 0).unwrap(); (addr_space_base, addr_space_base + addr_space_size) }; } ```

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SunriseOS/libuser/src/mem.rs

Lines 25 to 35 in 27ab3e4

	// TODO: Use svcGetInfo to get the address space in find_free_address
	// BODY: We should use svcGetInfo to get the address space in
	// BODY: `find_free_address`. This is extremely important as the low
	// BODY: addresses (from 0 to ADDRESS_SPACE_MIN) are not usable, but are
	// BODY: marked as available by QueryMemory.
	// BODY:
	// BODY: Here's a sample implementation for when we'll have GetInfo impl'd.
	// BODY:
	// BODY: ```rust
	// BODY: lazy_static! {
	// BODY: static ref ADDRESS_SPACE: (usize, usize) = {

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This issue was generated by todo based on a TODO comment in 27ab3e4 when #458 was merged. cc @roblabla.