Add `sigset64_t` and accompanying functions.

This doesn't address `struct sigaction` and `sigaction`. That will
come later.

Bug: http://b/72493232
Test: ran tests
Change-Id: I4134346757ce3a4dac6feae413361cec16223386
diff --git a/tests/Android.bp b/tests/Android.bp
index 25377df..443fe8e 100644
--- a/tests/Android.bp
+++ b/tests/Android.bp
@@ -140,6 +140,7 @@
         "sys_sem_test.cpp",
         "sys_sendfile_test.cpp",
         "sys_shm_test.cpp",
+        "sys_signalfd_test.cpp",
         "sys_socket_test.cpp",
         "sys_stat_test.cpp",
         "sys_statvfs_test.cpp",