[NFC][compiler-rt] Remove iOS xfail for unpoison-alternate-stack

This test was originally xfailed because of a bug on iOS. This has since been fixed, so reenabling the test.

(cherry picked from commit 05e48ec)

Author: aralisza

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

@@ -6,10 +6,6 @@
 // RUN: %clangxx_asan -std=c++20 -fexceptions -O0 %s -o %t -pthread
 // RUN: %run %t
 
-// XFAIL: ios && !iossim
-// longjmp from signal handler is unportable.
-// XFAIL: solaris
-
 #include <algorithm>
 #include <cassert>
 #include <cerrno>