[X86] Link safestacksepseg runtime

[mlemay-intel]

Summary

When SafeStack and the separate-stack-seg feature are both enabled, link the
safestacksepseg runtime library.

[mlemay-intel]

Only link safestacksepseg, not safestack. I changed the build configuration for safestacksepseg to also include the files in safestack.

[mlemay-intel]

Added test.

lib/Driver/Tools.cpp

if (SanArgs.needsTsanRt()) {
     if (SanArgs.linkCXXRuntimes())
       StaticRuntimes.push_back("tsan_cxx");
   }
   if (SanArgs.needsUbsanRt()) {
     StaticRuntimes.push_back("ubsan_standalone");
     if (SanArgs.linkCXXRuntimes())
       StaticRuntimes.push_back("ubsan_standalone_cxx");
   }
-  if (SanArgs.needsSafeStackRt())
+  if (SanArgs.needsSafeStackRt()) {
     StaticRuntimes.push_back("safestack");
+    if (Args.hasFlag(options::OPT_mseparate_stack_seg, options::OPT_mno_separate_stack_seg, false))
+      StaticRuntimes.push_back("safestacksepseg");
+  }
   if (SanArgs.needsCfiRt())
     StaticRuntimes.push_back("cfi");
   if (SanArgs.needsCfiDiagRt()) {
     StaticRuntimes.push_back("cfi_diag");
     if (SanArgs.linkCXXRuntimes())
       StaticRuntimes.push_back("ubsan_standalone_cxx");
   }
   if (SanArgs.needsStatsRt()) {