Use __cdecl rather than __stdcall for icalls on Windows 32-bit

Use __cdecl rather than __stdcall for icalls on Windows 32-bit

Mono assumes the same calling convention for icalls as the default P/Invoke
calling convention which is __stdcall on Windows. But none of the icalls are
marked with the required __stdcall attribute on Windows so there will be a
mismatch between the caller and callee.

This is the reason why the System.Web.HttpUtilityTest.JavaScriptStringEncode()
test crashes on Windows 32-bit. It generates a call to an icall
(char.ToString() inlines to new string(char,int)) which is assumed to be
__stdcall while it's actually __cdecl. The caller will decrement ESP as if the
callee was __stdcall resulting in ESP decreasing on each iteration. After a
couple of thousand iterations in the loop in that test the stack guard page is
hit and a stack overflow occurs.