Make unified headers' SIGRTMIN/SIGRTMAX usable before API 21.

Bug: android/ndk#352
Test: built new NDK test
Change-Id: Iacebe574bbf693701949e038005a40ba6520d592

libc/bionic/__libc_current_sigrtmax.cpp

@@ -29,5 +29,7 @@
 #include <signal.h>
 
 int __libc_current_sigrtmax(void) {
+  // If you change this, also change __ndk_legacy___libc_current_sigrtmax
+  // in <android/legacy_signal_inlines.h> to match.
   return __SIGRTMAX;
 }

libc/bionic/__libc_current_sigrtmin.cpp

@@ -34,5 +34,7 @@
 // __SIGRTMIN + 3 is reserved for triggering native stack dumps.
 
 int __libc_current_sigrtmin(void) {
+  // If you change this, also change __ndk_legacy___libc_current_sigrtmin
+  // in <android/legacy_signal_inlines.h> to match.
   return __SIGRTMIN + 4;
 }

libc/include/android/legacy_signal_inlines.h

@@ -41,6 +41,25 @@ sighandler_t bsd_signal(int signum, sighandler_t handler) __REMOVED_IN(21);
 
 #if __ANDROID_API__ < __ANDROID_API_L__
 
+/* These weren't introduced until L. */
+int __libc_current_sigrtmax() __attribute__((__weak__)) __VERSIONER_NO_GUARD;
+int __libc_current_sigrtmin() __attribute__((__weak__)) __VERSIONER_NO_GUARD;
+
+static __inline int __ndk_legacy___libc_current_sigrtmax() {
+  if (__libc_current_sigrtmax) return __libc_current_sigrtmax();
+  return __SIGRTMAX; /* Should match __libc_current_sigrtmax. */
+}
+
+static __inline int __ndk_legacy___libc_current_sigrtmin() {
+  if (__libc_current_sigrtmin) return __libc_current_sigrtmin();
+  return __SIGRTMIN + 4; /* Should match __libc_current_sigrtmin. */
+}
+
+#undef SIGRTMAX
+#define SIGRTMAX __ndk_legacy___libc_current_sigrtmax()
+#undef SIGRTMIN
+#define SIGRTMIN __ndk_legacy___libc_current_sigrtmin()
+
 static __inline int sigismember(const sigset_t *set, int signum) {
   /* Signal numbers start at 1, but bit positions start at 0. */
   int bit = signum - 1;