Feature request: implement Windows X86 thiscall

[gynt]

Basically, the first argument to a function specified as a thiscall (calling convention, attributes(thiscall)) should be placed into ECX, all other arguments are pushed to the stack, the callee does stack cleanup. It is a windows stdcall with the first argument passed to ecx. This feature would allow calling into C++ libraries.

// Calling this function:
attributes(thiscall) void f(void* this, int a, int b, int c);

// e.g., using this line: f((void*) 0xBEEFBEEF, 1, 2, 3)
// Should produce this code:
push 3
push 2
push 1
mov ecx, 0xBEEFBEEF 
call f // the callee cleans up the stack