[NFC][compiler-rt] add back solaris xfail for unpoison-alternate-stack.cpp

Author: aralisza

compiler-rt/test/asan/TestCases/Posix/unpoison-alternate-stack.cpp

@@ -6,6 +6,9 @@
 // RUN: %clangxx_asan -std=c++20 -fexceptions -O0 %s -o %t -pthread
 // RUN: %run %t
 
+// longjmp from signal handler is unportable.
+// XFAIL: solaris
+
 #include <algorithm>
 #include <cassert>
 #include <cerrno>