Refactor service requests

Refactor service requests

Service requests are part of the translator infrastructure and are
entirely[1] platform-agnostic. I factored out the definitions and the
argument-packing code into their own file.

Then I moved emitServiceReqWork into CodeGenHelpersX64. It was a little
difficult to untangle, since it had several direct uses of tx64 state. I
tried moving emitBindCall and friends too, but it got a lot more
difficult. I'll do that in another diff, to keep this one manageable.

I put this new header file in HPHP::JIT, so I had to introduce some
JIT:: line-noise elsewhere, but this is actually the lesser of two
evils. If I put it in HPHP::Transl, there ends up being Transl::
line-noise in a lot more places. Since JIT is where we're ultimately
headed to (right?), this seemed better.

[1] They're not entirely platform-agnostic, but they should be. Right
now they depend on the x64-specific ConditionCode enum. We use a very
limited set of x64 condition codes in this context, so we should be able
to define a platform-agnostic set of condition codes (a subset of the
intersection of ARM and x64 condition codes) and use it here. I'm
punting on that for now, though.

Differential Revision: D933753