this further reduces the number of source files which need to include libc.h and thereby be potentially exposed to libc global state and internals. this will also facilitate further improvements like adding an inline fast-path, if we want to do so later.
libc.h was intended to be a header for access to global libc state and related interfaces, but ended up included all over the place because it was the way to get the weak_alias macro. most of the inclusions removed here are places where weak_alias was needed. a few were recently introduced for hidden. some go all the way back to when libc.h defined CANCELPT_BEGIN and _END, and all (wrongly implemented) cancellation points had to include it. remaining spurious users are mostly callers of the LOCK/UNLOCK macros and files that use the LFS64 macro to define the awful *64 aliases. in a few places, new inclusion of libc.h is added because several internal headers no longer implicitly include libc.h. declarations for __lockfile and __unlockfile are moved from libc.h to stdio_impl.h so that the latter does not need libc.h. putting them in libc.h made no sense at all, since the macros in stdio_impl.h are needed to use them correctly anyway.
the malloc-implementation-private header is the only right place for this, because, being in the reserved namespace, __memalign is not interposable and thus not valid to use anywhere else. anything outside of the malloc implementation must call an appropriate-namespace public function (aligned_alloc or posix_memalign).
commit 618b18c78e33acfe54a4434e91aa57b8e171df89 removed the previous detection and hardening since it was incorrect. commit 72141795d4edd17f88da192447395a48444afa10 already handled all that remained for hardening the static-linked case. in the dynamic-linked case, have the dynamic linker check whether malloc was replaced and make that information available. with these changes, the properties documented in commit c9f415d7ea2dace5bf77f6518b6afc36bb7a5732 are restored: if calloc is not provided, it will behave as malloc+memset, and any of the memalign-family functions not provided will fail with ENOMEM.
this change serves multiple purposes: 1. it ensures that static linking of memalign-family functions will pull in the system malloc implementation, thereby causing link errors if an attempt is made to link the system memalign functions with a replacement malloc (incomplete allocator replacement). 2. it eliminates calls to free that are unpaired with allocations, which are confusing when setting breakpoints or tracing execution. as a bonus, making __bin_chunk external may discourage aggressive and unnecessary inlining of it.
the generated code should be mostly unchanged, except for explicit use of C_INUSE in place of copying the low bits from existing chunk headers/footers. these changes also remove mild UB due to dubious arithmetic on pointers into imaginary size_t[] arrays.