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