Don't use alternate stack on illumos or Solaris.

When .NET translates SIGSEV to NullReferenceException, it does not return
from the signal handler. Instead it resumes execution at the catch handler
for the exception. This is not recommend by the manpage for sigaction(2):

> It is not recommended that [the ucontext] arg be used by the handler to
> restore the context from before the signal delivery.

The practical effect of resuming execution without returning from a handler
is that the alternate stack will not be used for subsequent signal delivery.
This is in contrast to the behavior on linux, which will always use the
alternate stack if the stack pointer at the time of fault does not fall on
the alternate stack.

Since the alternate stack is only usable for a single exception, don't
bother using it for any exceptions.

src/coreclr/pal/src/exception/signal.cpp

@@ -190,8 +190,13 @@ BOOL SEHInitializeSignals(CorUnix::CPalThread *pthrCurrent, DWORD flags)
         handle_signal(SIGSEGV, sigsegv_handler, &g_previous_sigsegv);
 #else
         handle_signal(SIGTRAP, sigtrap_handler, &g_previous_sigtrap);
+        int additionalFlagsForSigSegv = 0;
+#ifndef TARGET_SUNOS
+        // On platforms that support signal handlers that don't return,
         // SIGSEGV handler runs on a separate stack so that we can handle stack overflow
-        handle_signal(SIGSEGV, sigsegv_handler, &g_previous_sigsegv, SA_ONSTACK);
+        additionalFlagsForSigSegv |= SA_ONSTACK;
+#endif
+        handle_signal(SIGSEGV, sigsegv_handler, &g_previous_sigsegv, additionalFlagsForSigSegv);
 
         if (!pthrCurrent->EnsureSignalAlternateStack())
         {
@@ -344,7 +349,7 @@ Return :
 --*/
 bool IsRunningOnAlternateStack(void *context)
 {
-#if HAVE_MACH_EXCEPTIONS
+#if HAVE_MACH_EXCEPTIONS || defined(TARGET_SUNOS)
     return false;
 #else
     bool isRunningOnAlternateStack;