Don't include custom chkstk on MSVC

MSVC includes its own __chkstk so these aren't used.

src/x86_64.rs

@@ -8,7 +8,7 @@ use core::intrinsics;
 // NOTE These functions are never mangled as they are not tested against compiler-rt
 // and mangling ___chkstk would break the `jmp ___chkstk` instruction in __alloca
 
-#[cfg(windows)]
+#[cfg(all(windows, target_env = "gnu"))]
 #[naked]
 #[cfg_attr(not(feature = "mangled-names"), no_mangle)]
 pub unsafe fn ___chkstk_ms() {
@@ -32,7 +32,7 @@ pub unsafe fn ___chkstk_ms() {
     intrinsics::unreachable();
 }
 
-#[cfg(windows)]
+#[cfg(all(windows, target_env = "gnu"))]
 #[naked]
 #[cfg_attr(not(feature = "mangled-names"), no_mangle)]
 pub unsafe fn __alloca() {
@@ -41,7 +41,7 @@ pub unsafe fn __alloca() {
     intrinsics::unreachable();
 }
 
-#[cfg(windows)]
+#[cfg(all(windows, target_env = "gnu"))]
 #[naked]
 #[cfg_attr(not(feature = "mangled-names"), no_mangle)]
 pub unsafe fn ___chkstk() {