Merge pull request swiftlang#14898 from compnerd/aarch64-size-shims

Author: swift-ci

stdlib/public/SwiftShims/LibcShims.h

@@ -38,7 +38,7 @@ typedef           int __swift_ssize_t;
 #elif defined(_WIN32)
 #if defined(_M_ARM) || defined(_M_IX86)
 typedef           int __swift_ssize_t;
-#elif defined(_M_X64)
+#elif defined(_M_X64) || defined(_M_ARM64)
 typedef long long int __swift_ssize_t;
 #else
 #error unsupported machine type