asan: fix crash on odd stack size

[dvyukov]

Summary

The test currently crashes as:

AddressSanitizer: CHECK failed: asan_poisoning.cpp:38 "((AddrIsAlignedByGranularity(addr))) != (0)"

Main stack address/size don't have to be aligned on asan shadow granularity.
Align stack bottom.

[vitalybuka]

This one is flaky on power pc
https://lab.llvm.org/buildbot/#/builders/19
https://lab.llvm.org/buildbot/#/builders/18

[dvyukov]

In D145799#4187988, @vitalybuka wrote:

This one is flaky on power pc
https://lab.llvm.org/buildbot/#/builders/19
https://lab.llvm.org/buildbot/#/builders/18

Sent https://reviews.llvm.org/D145902

compiler-rt/lib/asan/asan_thread.cpp

 // OS-specific implementations that need more information passed through.
 void AsanThread::SetThreadStackAndTls(const InitOptions *options) {
   DCHECK_EQ(options, nullptr);
   uptr tls_size = 0;
   uptr stack_size = 0;
   GetThreadStackAndTls(tid() == kMainTid, &stack_bottom_, &stack_size,
                        &tls_begin_, &tls_size);
   stack_top_ = RoundDownTo(stack_bottom_ + stack_size, ASAN_SHADOW_GRANULARITY);
+  stack_bottom_ = RoundDownTo(stack_bottom_, ASAN_SHADOW_GRANULARITY);
   tls_end_ = tls_begin_ + tls_size;
   dtls_ = DTLS_Get();
 
   if (stack_top_ != stack_bottom_) {
     int local;
     CHECK(AddrIsInStack((uptr)&local));
   }
 }

compiler-rt/test/sanitizer_common/TestCases/Linux/odd_stack_size.cpp

+// RUN: %clangxx -O1 %s -o %t && %run %t
+// UNSUPPORTED: android
+
+#include <assert.h>
+#include <stdlib.h>
+#include <sys/resource.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+int main(int argc, char **argv) {
+  if (getenv("SANITIZER_TEST_REEXECED"))
+    exit(0);
+  struct rlimit rl;
+  assert(!getrlimit(RLIMIT_STACK, &rl));
+  struct rlimit rl_new = rl;
+  rl_new.rlim_cur = 17351;
+  assert(!setrlimit(RLIMIT_STACK, &rl_new));
+  int pid = fork();
+  assert(pid >= 0);
+  if (pid == 0) {
+    const char *envp[] = {"SANITIZER_TEST_REEXECED=1", nullptr};
+    execve(argv[0], argv, const_cast<char **>(envp));
+    assert(false);
+  }
+  int status;
+  while (waitpid(-1, &status, __WALL) != pid) {
+  }
+  assert(WIFEXITED(status) && WEXITSTATUS(status) == 0);
+  return 0;
+}