[asan][test] Several Posix/unpoison-alternate-stack.cpp fixes

`Posix/unpoison-alternate-stack.cpp` currently `FAIL`s on Solaris/i386.
Some of the problems are generic:

- `clang` warns compiling the testcase:

  compiler-rt/test/asan/TestCases/Posix/unpoison-alternate-stack.cpp:83:7: warning: nested designators are a C99 extension [-Wc99-designator]
        .sa_sigaction = signalHandler,
        ^~~~~~~~~~~~~
  compiler-rt/test/asan/TestCases/Posix/unpoison-alternate-stack.cpp:84:7: warning: ISO C++ requires field designators to be specified in declaration order; field '_funcptr' will be initialized after field 'sa_flags' [-Wreorder-init-list]
        .sa_flags = SA_SIGINFO | SA_NODEFER | SA_ONSTACK,
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

  and some more instances.  This can all easily be avoided by initializing
  each field separately.

- The test `SEGV`s in `__asan_memcpy`.  The default Solaris/i386 stack size
  is only 4 kB, while `__asan_memcpy` tries to allocate either 5436
  (32-bit) or 10688 bytes (64-bit) on the stack.  This patch avoids this by
  requiring at least 16 kB stack size.

- Even without `-fsanitize=address` I get an assertion failure:

  Assertion failed: !isOnSignalStack(), file compiler-rt/test/asan/TestCases/Posix/unpoison-alternate-stack.cpp, line 117

  The fundamental problem with this testcase is that `longjmp` from a
  signal handler is highly unportable; XPG7 strongly warns against it and
  it is thus unspecified which stack is used when `longjmp`ing from a
  signal handler running on an alternative stack.

  So I'm `XFAIL`ing this testcase on Solaris.

Tested on `amd64-pc-solaris2.11` and `x86_64-pc-linux-gnu`.

Differential Revision: https://reviews.llvm.org/D88501

Author: rorth

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

@@ -7,7 +7,10 @@
 // RUN: %run %t
 
 // XFAIL: ios && !iossim
+// longjmp from signal handler is unportable.
+// XFAIL: solaris
 
+#include <algorithm>
 #include <cassert>
 #include <cerrno>
 #include <csetjmp>
@@ -83,10 +86,9 @@ void signalHandler(int, siginfo_t *, void *) {
 void setSignalAlternateStack(void *AltStack) {
   sigaltstack((stack_t const *)AltStack, nullptr);
 
-  struct sigaction Action = {
-      .sa_sigaction = signalHandler,
-      .sa_flags = SA_SIGINFO | SA_NODEFER | SA_ONSTACK,
-  };
+  struct sigaction Action = {};
+  Action.sa_sigaction = signalHandler;
+  Action.sa_flags = SA_SIGINFO | SA_NODEFER | SA_ONSTACK;
   sigemptyset(&Action.sa_mask);
 
   sigaction(SIGUSR1, &Action, nullptr);
@@ -137,9 +139,11 @@ void *threadFun(void *AltStack) {
 // reports when the stack is reused.
 int main() {
   size_t const PageSize = sysconf(_SC_PAGESIZE);
+  // The Solaris defaults of 4k (32-bit) and 8k (64-bit) are too small.
+  size_t const MinStackSize = std::max(PTHREAD_STACK_MIN, 16 * 1024);
   // To align the alternate stack, we round this up to page_size.
   size_t const DefaultStackSize =
-      (PTHREAD_STACK_MIN - 1 + PageSize) & ~(PageSize - 1);
+      (MinStackSize - 1 + PageSize) & ~(PageSize - 1);
   // The alternate stack needs a certain size, or the signal handler segfaults.
   size_t const AltStackSize = 10 * PageSize;
   size_t const MappingSize = DefaultStackSize + AltStackSize;
@@ -149,11 +153,10 @@ int main() {
                              MAP_PRIVATE | MAP_ANONYMOUS,
                              -1, 0);
 
-  stack_t const AltStack = {
-      .ss_sp = (char *)Mapping + DefaultStackSize,
-      .ss_flags = 0,
-      .ss_size = AltStackSize,
-  };
+  stack_t AltStack = {};
+  AltStack.ss_sp = (char *)Mapping + DefaultStackSize;
+  AltStack.ss_flags = 0;
+  AltStack.ss_size = AltStackSize;
 
   pthread_t Thread;
   pthread_attr_t ThreadAttr;