[musl] Remove LFS64 and _LARGEFILE64_SOURCE definitions

This was reverted previously as dart was using some of these
definitions. Dart has since been changed.

Test: full build, plus grep

Change-Id: Ibce5d9873b6132e5099370a5be7ddd54e7009aa2

[musl] Reintroduce LFS64 definitions

This partially reverts commit
e831a8265b4c3954eb78782c24ce5257520255b2.

These are currently used by the dart runtime. Once it is patched not
to do so, they will be removed again.

Test: built upper layers, including dart

Change-Id: I8261dd44c3ff0ceb067395afe76cb80f4c6f5a8e

[musl] Remove LFS64 definitions

Test: dash was the only client, and the shell still works

Change-Id: Ib5c2b99634b7b3545ef621096fe33663aadfb99f

[musl] Use #pragma once in musl's public headers

Like the prior commit, this only replaces existing ifdef guards.

Change-Id: I39be129111db9f129aef72a53f986ce6a49376ac

[magenta] Initial commit