memrchr: Correct aligned offset computation

The memrchr fallback did not compute the offset correctly. It was
intentioned to land on usize-aligned addresses but did not.
This was suspected to resulted in a crash on ARMv7 platform!

This bug affected non-linux platforms.

I think like this, if we have a slice with pointer `ptr` and length
`len`, we want to find the last usize-aligned offset in the slice.
The correct computation should be:

For example if ptr = 1 and len = 6, and size_of::<usize>() is 4:

[ x x x x x x ]
  1 2 3 4 5 6
        ^-- last aligned address at offset 3 from the start.

The last aligned address is ptr + len - (ptr + len) % usize_size.

Compute offset from the start as:

offset = len - (ptr + len) % usize_size = 6 - (1 + 6) % 4 = 6 - 3 = 3.

I believe the function's return value was always correct previously, if
the platform supported unaligned addresses.

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