Allow the mingw prebuilt to access stdatomic.h

Add a symbolic link from the mingw prebuilt to the bionic stdatomic.h.

Change-Id: Id2c863d96b1ea7ff7b4fc54c3b245e36f0145337
diff --git a/x86_64-w64-mingw32/include/stdatomic.h b/x86_64-w64-mingw32/include/stdatomic.h
new file mode 120000
index 0000000..da186e2
--- /dev/null
+++ b/x86_64-w64-mingw32/include/stdatomic.h
@@ -0,0 +1 @@
+../../../../../../../bionic/libc/include/stdatomic.h
\ No newline at end of file