signal: make sig_atomic_t a normal int

Using the _Atomic types is technically more accurate, but equivalent in
practice (glibc uses a normal int) and a bit of a headache for C++
targets since _Atomic is replaced with std::atomic.

include/signal.h

@@ -7,11 +7,10 @@ extern "C" {
 
 #include <stddef.h>
 #include <stdint.h>
-#include <stdatomic.h>
 
 /* C99 API. */
 
-typedef volatile atomic_int sig_atomic_t;
+typedef int sig_atomic_t;
 
 /* Type of a signal handler.*/
 typedef void (*__sighandler_t)(int);