[_AtomicShims] On Darwin, ensure we link against libswiftCore using assembly shenanigans

This avoids a linker failure when the build system decides to build this module as a standalone library.

Hopefully this will all go away soon, when swift_retain_n/swift_release_n will become primitives exposed by the stdlib.

Author: lorentey

Sources/_AtomicsShims/src/_AtomicsShims.c

@@ -16,6 +16,14 @@
 // like calls to swift_retain_n/swift_release_n appearing in Swift code, not
 // even when imported through C. (See https://bugs.swift.org/browse/SR-13708)
 
+#if defined(__APPLE__) && defined(__MACH__)
+// Ensure we link with libswiftCore.dylib even when the build system decides
+// to build this module as a standalone library.
+// (See https://github.com/apple/swift-atomics/issues/55)
+__asm__(".linker_option \"-L/usr/lib/swift\"\n");
+__asm__(".linker_option \"-lswiftCore\"\n");
+#endif
+
 void _sa_retain_n(void *object, uint32_t n)
 {
   extern void *swift_retain_n(void *object, uint32_t n);