Use timelib_diff to calculate datetime intervals

Summary: Fixes #2952

Reviewed By: @JoelMarcey

Differential Revision: D1657306

Signature: t1:1657306:1415077618:3781defe888af49033d1743a7d50aa48612141a0
Convert a few ARM opcodes to DELEGATE

Summary: This is just a few of the easiest ones, to get started with. It helped
to flush out a few issues:

- x64's implementation of DefSP was referring to rbp explicitly, instead
  of the tmp representing the frame pointer.

- ARM was defining a status flag register twice; this looks like a rough
  edge of the conversion to explicit status flags.

Reviewed By: @edwinsmith

Differential Revision: D1658070