[sanitizer] Minor refactor of ThreadDescriptorSize

[cryptoad]

Summary

While I was sifting through dead code findings, I stumbled on this function.

First, for __s390__ it always returned 0 for the 1st call, which doesn't seem
right. 2nd call & beyond would return the correct value though.
Then it duplicated the atomic_store multiple times, sometimes with a if,
sometimes without. Finally it used a capitalized variable name starting with k
which indicates a constant, and it is not.

So:

• rename the static global variable;
• change the atomic functions to their relaxed version;
• move the store to the end, and make sure we return val all the time.

lib/sanitizer_common/sanitizer_linux_libcdep.cc

 #endif  // !SANITIZER_FREEBSD && !SANITIZER_ANDROID && !SANITIZER_GO &&
         // !SANITIZER_NETBSD && !SANITIZER_SOLARIS
 
 #if (defined(__x86_64__) || defined(__i386__) || defined(__mips__) ||          \
      defined(__aarch64__) || defined(__powerpc64__) || defined(__s390__) ||    \
      defined(__arm__)) &&                                                      \
     SANITIZER_LINUX && !SANITIZER_ANDROID
 // sizeof(struct pthread) from glibc.
-static atomic_uintptr_t kThreadDescriptorSize;
+static atomic_uintptr_t thread_descriptor_size;
 
 uptr ThreadDescriptorSize() {
-  uptr val = atomic_load(&kThreadDescriptorSize, memory_order_relaxed);
+  uptr val = atomic_load_relaxed(&thread_descriptor_size);
   if (val)
     return val;
 #if defined(__x86_64__) || defined(__i386__) || defined(__arm__)
 #ifdef _CS_GNU_LIBC_VERSION
   char buf[64];
   uptr len = confstr(_CS_GNU_LIBC_VERSION, buf, sizeof(buf));
   if (len < sizeof(buf) && internal_strncmp(buf, "glibc 2.", 8) == 0) {
     char *end;
if (end != buf + 8 && (*end == '\0' || *end == '.' || *end == '-')) {
         val = FIRST_32_SECOND_64(1168, 1776);
       else if (minor == 11 || (minor == 12 && patch == 1))
         val = FIRST_32_SECOND_64(1168, 2288);
       else if (minor <= 13)
         val = FIRST_32_SECOND_64(1168, 2304);
       else
         val = FIRST_32_SECOND_64(1216, 2304);
     }
-    if (val)
-      atomic_store(&kThreadDescriptorSize, val, memory_order_relaxed);
-    return val;
   }
 #endif
 #elif defined(__mips__)
   // TODO(sagarthakur): add more values as per different glibc versions.
   val = FIRST_32_SECOND_64(1152, 1776);
-  if (val)
-    atomic_store(&kThreadDescriptorSize, val, memory_order_relaxed);
-  return val;
 #elif defined(__aarch64__)
   // The sizeof (struct pthread) is the same from GLIBC 2.17 to 2.22.
   val = 1776;
-  atomic_store(&kThreadDescriptorSize, val, memory_order_relaxed);
-  return val;
 #elif defined(__powerpc64__)
   val = 1776; // from glibc.ppc64le 2.20-8.fc21
-  atomic_store(&kThreadDescriptorSize, val, memory_order_relaxed);
-  return val;
 #elif defined(__s390__)
   val = FIRST_32_SECOND_64(1152, 1776); // valid for glibc 2.22
-  atomic_store(&kThreadDescriptorSize, val, memory_order_relaxed);
 #endif
-  return 0;
+  if (val)
+    atomic_store_relaxed(&thread_descriptor_size, val);
+  return val;
 }
 
 // The offset at which pointer to self is located in the thread descriptor.
 const uptr kThreadSelfOffset = FIRST_32_SECOND_64(8, 16);
 
 uptr ThreadSelfOffset() {
   return kThreadSelfOffset;
 }