Reserve realtime signal for libprofile-extras

Bug: http://b/128524141

This signal is used by libprofile-extras to trigger flush of gcov
coverage data.

Test: libprofile-extras in system/extras/toolchain-extras
Change-Id: I77dd5b1d67371a9cfaad54ad60254bfb2dd18ba9
diff --git a/libc/private/sigrtmin.h b/libc/private/sigrtmin.h
index 67bd864..d78d980 100644
--- a/libc/private/sigrtmin.h
+++ b/libc/private/sigrtmin.h
@@ -40,11 +40,12 @@
 //   34 (__SIGRTMIN + 2)        libcore
 //   35 (__SIGRTMIN + 3)        debuggerd -b
 //   36 (__SIGRTMIN + 4)        heapprofd
+//   37 (__SIGRTMIN + 5)        coverage (libprofile-extras)
 //
 // If you change this, also change __ndk_legacy___libc_current_sigrtmin
 // in <android/legacy_signal_inlines.h> to match.
 
-#define __SIGRT_RESERVED 5
+#define __SIGRT_RESERVED 6
 static inline __always_inline sigset64_t filter_reserved_signals(sigset64_t sigset, int how) {
   int (*block)(sigset64_t*, int);
   int (*unblock)(sigset64_t*, int);
@@ -70,5 +71,6 @@
   unblock(&sigset, __SIGRTMIN + 2);
   unblock(&sigset, __SIGRTMIN + 3);
   unblock(&sigset, __SIGRTMIN + 4);
+  unblock(&sigset, __SIGRTMIN + 5);
   return sigset;
 }