libc_shared_globals: add a constexpr ctor

Having a constexpr constructor should guarantee that the static
`globals` variable in __libc_shared_globals is initialized statically
(as opposed to dynamically), which is important because
__libc_shared_globals is called very early (before the linker has
relocated itself). With the constructor, though, the fields can safely
have in-line default initializers.

Bug: none
Test: bionic unit tests
Change-Id: Icde821557369625734a4d85d7ff55428bad5c247
diff --git a/libc/private/bionic_fdsan.h b/libc/private/bionic_fdsan.h
index de14cf8..f403d08 100644
--- a/libc/private/bionic_fdsan.h
+++ b/libc/private/bionic_fdsan.h
@@ -39,21 +39,23 @@
 #include <sys/user.h>
 
 struct FdEntry {
-  _Atomic(uint64_t) close_tag;
+  _Atomic(uint64_t) close_tag = 0;
 };
 
 struct FdTableOverflow {
-  size_t len;
+  size_t len = 0;
   FdEntry entries[0];
 };
 
 template <size_t inline_fds>
 struct FdTableImpl {
-  uint32_t version;  // currently 0, and hopefully it'll stay that way.
-  _Atomic(android_fdsan_error_level) error_level;
+  constexpr FdTableImpl() {}
+
+  uint32_t version = 0;  // currently 0, and hopefully it'll stay that way.
+  _Atomic(android_fdsan_error_level) error_level = ANDROID_FDSAN_ERROR_LEVEL_DISABLED;
 
   FdEntry entries[inline_fds];
-  _Atomic(FdTableOverflow*) overflow;
+  _Atomic(FdTableOverflow*) overflow = nullptr;
 
   FdEntry* at(size_t idx);
 };