XTF: Use fastcall by default for 32bit

XTF: Use fastcall by default for 32bit

This passes up to 3 parameters in registers, rather than on the stack.  Most
transformations are easy.

The exec_user() infrastructure took two parameters on the stack and used an
ad-hoc %eax/%ecx arrangement with %edx as a scratch register. Fastcall uses
%eax/%edx, so switch the scratch regsiter to %ecx instead.

memop-seg was already using %eax, and needs a bit of care now that addr isn't
on the stack.  However, it does remove the need for stack_adj which simplifies
the result.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>