Minor fixes and cleaning of arm64 static and direct calls code.

The proper way to avoid the MacroAssembler to generate code before or
after an instruction is to block the pools (usually via
`vixl::BlockPoolsScope`). Here we can use
`vixl::SingleEmissionCheckScope`, that checks we generate only one
instruction and also blocks the pools.
In practice the current code would have worked fine because VIXL would
not have generated anything after `Bl()` or `Ldr()`, but that was not

- `XRegisterFrom()` returns an X register. Calling `.X()` is not
- Since we are sure (after the previous fixes) that nothing will be
  emitted around the instructions we care about, update the code to
  bind labels before the instructions for simplicity.

Change-Id: I42d49976721e380e66bcd7a5b345f1777009434a
2 files changed