windows fastcall (x64) call convention (#314)

* initial set of work for windows fastcall (x64) call convention

- call conventions: rename `fastcall` to `windows_fastcall`
- add initial set of filetests
- ensure arguments are written after the shadow space/store (offset-wise)
  The shadow space available before the arguments (range 0..32)
  is not used as spill space yet.

* address review feedback

lib/simplejit/src/backend.rs

@@ -9,6 +9,8 @@ use cretonne_native;
 use std::ffi::CString;
 use std::ptr;
 use libc;
+#[cfg(windows)]
+use winapi;
 use memory::Memory;
 
 /// A builder for `SimpleJITBackend`.
@@ -344,6 +346,7 @@ impl<'simple_jit_backend> Backend for SimpleJITBackend {
     fn finish(self) -> () {}
 }
 
+#[cfg(not(windows))]
 fn lookup_with_dlsym(name: &str) -> *const u8 {
     let c_str = CString::new(name).unwrap();
     let c_str_ptr = c_str.as_ptr();
@@ -354,6 +357,38 @@ fn lookup_with_dlsym(name: &str) -> *const u8 {
     sym as *const u8
 }
 
+#[cfg(windows)]
+fn lookup_with_dlsym(name: &str) -> *const u8 {
+    const MSVCRT_DLL: &[u8] = b"msvcrt.dll\0";
+
+    let c_str = CString::new(name).unwrap();
+    let c_str_ptr = c_str.as_ptr();
+
+    unsafe {
+        let handles = [
+            // try to find the searched symbol in the currently running executable
+            ptr::null_mut(),
+            // try to find the searched symbol in local c runtime
+            winapi::um::libloaderapi::GetModuleHandleA(MSVCRT_DLL.as_ptr() as *const i8),
+        ];
+
+        for handle in &handles {
+            let addr = winapi::um::libloaderapi::GetProcAddress(*handle, c_str_ptr);
+            if addr.is_null() {
+                continue;
+            }
+            return addr as *const u8;
+        }
+
+        let msg = if handles[1].is_null() {
+            "(msvcrt not loaded)"
+        } else {
+            ""
+        };
+        panic!("cannot resolve address of symbol {} {}", name, msg);
+    }
+}
+
 struct SimpleJITRelocSink {
     pub relocs: Vec<RelocRecord>,
 }