Shims: strongly link swift_isStackAllocationSafe on !Darwin

Windows objects to the multiple different aliases for
`swift_isStackAllocationSafe`.  Use strong linkage on all non-ABI stable
targets.  This should repair the Windows build and matches the
conditional emission in the stdlib.

Thanks to @lorentey for the discussion around how to handle this and the
pointer to where this was getting introduced!

Author: compnerd

stdlib/public/SwiftShims/RuntimeShims.h

@@ -93,7 +93,10 @@ __swift_size_t _swift_stdlib_getHardwareConcurrency(void);
 ///
 /// This function is used by
 /// @c withUnsafeTemporaryAllocation(byteCount:alignment:_:).
-SWIFT_RUNTIME_STDLIB_API SWIFT_WEAK_IMPORT
+SWIFT_RUNTIME_STDLIB_API
+#if defined(__APPLE__) && defined(__MACH__)
+SWIFT_WEAK_IMPORT
+#endif
 __swift_bool swift_stdlib_isStackAllocationSafe(__swift_size_t byteCount,
                                                 __swift_size_t alignment);