Make AtomicsShims compatible with C++ interop

This fixes linker errors such as
```
Undefined symbols for architecture arm64:
  "_sa_retain_n(void*, unsigned int)", referenced from:
      (extension in Atomics):Swift.Unmanaged.retain(by: Swift.Int) -> () in Unmanaged extensions.swift.o
   NOTE: found '__sa_retain_n' in _AtomicsShims.c.o, declaration possibly missing 'extern "C"'
```
when building with Swift/C++ interoperability enabled.

Author: egorzhdan

Sources/_AtomicsShims/include/_AtomicsShims.h

@@ -34,7 +34,11 @@
 #    define SWIFTATOMIC_SHIMS_EXPORT
 #  endif
 #else
-#  define SWIFTATOMIC_SHIMS_EXPORT extern
+#  ifdef __cplusplus
+#    define SWIFTATOMIC_SHIMS_EXPORT extern "C"
+#  else
+#    define SWIFTATOMIC_SHIMS_EXPORT extern
+#  endif
 #endif
 
 // Swift-importable shims for C atomics.