Add __chkstk on i686-pc-windows-gnu.

libLLVMSupport.a(DynamicLibrary.cpp.obj) references ___chkstk, which is
an alias of __alloca in libgcc.  This crate provided __alloca, but
libgcc's implementation was also pulled in by the linker due to the
reference to ___chkstk, causing a multiple definition linker error.
Providing that symbol here prevents that.

Fixes #585

library/compiler-builtins/src/x86.rs

@@ -8,6 +8,19 @@ use core::intrinsics;
 // NOTE These functions are never mangled as they are not tested against compiler-rt
 
 intrinsics! {
+    #[naked]
+    #[cfg(all(
+        windows,
+        target_env = "gnu",
+        not(feature = "no-asm")
+    ))]
+    pub unsafe extern "C" fn __chkstk() {
+        core::arch::asm!(
+            "jmp __alloca", // Jump to __alloca since fallthrough may be unreliable"
+            options(noreturn, att_syntax)
+        );
+    }
+
     #[naked]
     #[cfg(all(
         windows,
@@ -15,7 +28,7 @@ intrinsics! {
         not(feature = "no-asm")
     ))]
     pub unsafe extern "C" fn _alloca() {
-        // _chkstk and _alloca are the same function
+        // __chkstk and _alloca are the same function
         core::arch::asm!(
             "push   %ecx",
             "cmp    $0x1000,%eax",