atomicvar.h: show used API in INFO. Add macro to force __sync builtin.

The __sync builtin can be correctly detected by Helgrind so to force it
is useful for testing. The API in the INFO output can be useful for
debugging after problems are reported.

src/server.c

@@ -2827,6 +2827,7 @@ sds genRedisInfoString(char *section) {
             "os:%s %s %s\r\n"
             "arch_bits:%d\r\n"
             "multiplexing_api:%s\r\n"
+            "atomicvar_api:%s\r\n"
             "gcc_version:%d.%d.%d\r\n"
             "process_id:%ld\r\n"
             "run_id:%s\r\n"
@@ -2845,6 +2846,7 @@ sds genRedisInfoString(char *section) {
             name.sysname, name.release, name.machine,
             server.arch_bits,
             aeGetApiName(),
+            REDIS_ATOMIC_API,
 #ifdef __GNUC__
             __GNUC__,__GNUC_MINOR__,__GNUC_PATCHLEVEL__,
 #else