Avoid using glibc internals for SIGSYS handler

SIGSYS fields were added to siginfo_t on glibc in version 2.17,
which is quite old now. Additionally, musl libc uses a different
name (__si_fields) for its internal struct members.

To fix this, use the libc-defined members if they are available,
otherwise fall back to _sifields, as is done currently.

Change-Id: I6a542a1817df7e31309a1043b899328a50834f7d
Bug: 1045069
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2015735Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734970}

sandbox/linux/seccomp-bpf/trap.cc

@@ -164,11 +164,18 @@ void Trap::SigSys(int nr, LinuxSigInfo* info, ucontext_t* ctx) {
   }
 
 
-  // Obtain the siginfo information that is specific to SIGSYS. Unfortunately,
-  // most versions of glibc don't include this information in siginfo_t. So,
-  // we need to explicitly copy it into a arch_sigsys structure.
+  // Obtain the siginfo information that is specific to SIGSYS.
   struct arch_sigsys sigsys;
+#if defined(si_call_addr) && !defined(__native_client_nonsfi__)
+  sigsys.ip = info->si_call_addr;
+  sigsys.nr = info->si_syscall;
+  sigsys.arch = info->si_arch;
+#else
+  // If the version of glibc doesn't include this information in
+  // siginfo_t (older than 2.17), we need to explicitly copy it
+  // into an arch_sigsys structure.
   memcpy(&sigsys, &info->_sifields, sizeof(sigsys));
+#endif
 
 #if defined(__mips__)
   // When indirect syscall (syscall(__NR_foo, ...)) is made on Mips, the