Fix stack homing of register parameter offsets bugs.

Fix stack homing of register parameter offsets bugs.
This set of changes contain:
1. Fix of a RA conservative assertion. In case where there are no
callee-save registers the RA was asserting this fact. It is a valid state
on System V machines for XMM registers. The RA does spill and frees a
register if the assertion doesn't trigger. The fix is to make the assert
allow non-callee-saved registers.
2. It fixes a problem with hardcoding registers when exception funclets
are generated. It uses platform independent RBM_ARG_0 macros now.
3. It disables struct promotion for UNIX. This functionality will be
brought up later.
4. It addresses feedback from a previous PR.