signal.h: add new sa_flags from linux v5.11

see

  linux commit a54f0dfda754c5cecc89a14dab68a3edc1e497b5
  signal: define the SA_UNSUPPORTED bit in sa_flags

  linux commit 6ac05e832a9e96f9b1c42a8917cdd317d7b6c8fa
  signal: define the SA_EXPOSE_TAGBITS bit in sa_flags

Note: SA_ is in the posix reserved namespace so these linux specific flags
can be exposed when compiling for posix.

include/signal.h

@@ -178,6 +178,9 @@ struct sigaction {
 #define sa_handler   __sa_handler.sa_handler
 #define sa_sigaction __sa_handler.sa_sigaction
 
+#define SA_UNSUPPORTED 0x00000400
+#define SA_EXPOSE_TAGBITS 0x00000800
+
 struct sigevent {
 	union sigval sigev_value;
 	int sigev_signo;