Redo libc++ customizations

• Remove osreldate include because _LIBCPP_HAS_NO_GETS has disappeared
• Instead, add direct major FreeBSD check for using ::gets declaration
• Mark EINTEGRITY values as FreeBSD customization
• Reformat _LIBCPP_TYPE_VISIBILITY_DEFAULT customization

PR: 276104
MFC after: 1 month

(cherry picked from commit 46c59ea9b61755455ff6bf9f3e7b834e1af634ea)