Support special calling convention for runtime ABI
Support special calling convention for runtime ABI

Add infrastructure to support special calling convention for runtime ABI
function no matter what is the current calling convention. This involve
2 changes:
- behave as per base standard in gfunc_call
- move result back in VFP register in gen_cvt_itof1

mingodad/tinycc,seyko2/tinycc,metaeducation/tcc,fstudio/tinycc,leeduhem/tinycc,run4flat/tinycc,run4flat/tinycc,metaeducation/tcc,gfwilliams/tinycc,rchandramouli/tinycc,Saectar/inherpreter,mirror/tinycc,code4game/tinycc,Saectar/inherpreter,gfwilliams/tinycc,code4game/tinycc,andrewchambers/tccmirror,rchandramouli/tinycc,Saectar/inherpreter,alfonsokim/tinycc,andreiw/tinycc,gfwilliams/tinycc,andrewchambers/tccmirror,fstudio/tinycc,mirror/tinycc,leeduhem/tinycc,andrewchambers/tccmirror,gfwilliams/tinycc,alfonsokim/tinycc,seyko2/tinycc,andrewchambers/tccmirror,leeduhem/tinycc,mirror/tinycc,avih/tinycc,rchandramouli/tinycc,alfonsokim/tinycc,code4game/tinycc,seyko2/tinycc,fstudio/tinycc,mingodad/tinycc,andreiw/tinycc,Saectar/inherpreter,seyko2/tinycc,fstudio/tinycc,rchandramouli/tinycc,avih/tinycc,Saectar/inherpreter,andrewchambers/tccmirror,run4flat/tinycc,mingodad/tinycc,leeduhem/tinycc,leeduhem/tinycc,seyko2/tinycc,metaeducation/tcc,alfonsokim/tinycc,mingodad/tinycc,run4flat/tinycc,fstudio/tinycc,andreiw/tinycc,alfonsokim/tinycc,metaeducation/tcc,run4flat/tinycc,rchandramouli/tinycc,metaeducation/tcc,avih/tinycc,mirror/tinycc,gfwilliams/tinycc,avih/tinycc