[libc] Move preinit/init/fini arrays to namespace (#158746)

In change #146863 we moved definitions of preinit/init/fini arrays to
header but unintentionally moved outside of the namespace. Since the
namespace also controls the visibility (through LIBC_NAMESPACE_DECL), as
a consequence these symbols no longer have the hidden visibility which
changes the codegen from:

```
 4: 4c11          ldr     r4, [pc, #0x44]         @ 0x4c <__libc_init_array+0x4c>
 6: 4812          ldr     r0, [pc, #0x48]         @ 0x50 <__libc_init_array+0x50>
 8: 447c          add     r4, pc
 a: 4478          add     r0, pc
 c: 1b00          subs    r0, r0, r4
```

to:

```
 4: 4813          ldr     r0, [pc, #0x4c]         @ 0x54 <__libc_init_array+0x54>
 6: 4914          ldr     r1, [pc, #0x50]         @ 0x58 <__libc_init_array+0x58>
 8: 4478          add     r0, pc
 a: 4479          add     r1, pc
 c: 6804          ldr     r4, [r0]
 e: 6808          ldr     r0, [r1]
10: 1b00          subs    r0, r0, r4
```

The `ldr` will trigger a fault in case where these symbols aren't
pointing to a valid memory location which is sometimes the case when the
array is empty.