Reland "sigchain: add wrappers for sigaction64/sigprocmask64."

This reverts commit bed29657743f64612be4969f036c324df4baf7a9.

"Fix" the mac build by using sigchain_dummy on it unconditionally.

Also, fix a test breakage caused by use of memcmp on a struct with
padding bytes that went from being assigned via memcpy to manual
assignment of each of the fields.

Test: test/testrunner/testrunner.py -b --target -t 004-SignalTest
Change-Id: I332d633a06abbf189d1e51fa8d031c2745f8f1a7
diff --git a/sigchainlib/version-script64.txt b/sigchainlib/version-script64.txt
index acf3630..72c86a1 100644
--- a/sigchainlib/version-script64.txt
+++ b/sigchainlib/version-script64.txt
@@ -4,8 +4,10 @@
   AddSpecialSignalHandlerFn;
   RemoveSpecialSignalHandlerFn;
   sigaction;
+  sigaction64;
   signal;
   sigprocmask;
+  sigprocmask64;
 local:
   *;
 };