Note [Caller saves and callee-saves regs.]

Caller-saves regs have to be saved around C-calls made from STG
land, so this file defines CALLER_SAVES_<reg> for each <reg> that
is designated caller-saves in that machine's C calling convention.
NB: Caller-saved registers not mapped to a STG register don't
    require a CALLER_SAVES_ define.

As it stands, the only registers that are ever marked caller saves
are the RX, FX, DX, XMM and USER registers; as a result, if you
decide to caller save a system register (e.g. SP, HP, etc), note that
this code path is completely untested! -- EZY

See Note [Register parameter passing] for details.