reduce spurious inclusion of libc.h

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.

src/fenv/fesetround.c

@@ -1,5 +1,5 @@
 #include <fenv.h>
-#include "libc.h"
+#include <features.h>
 
 /* __fesetround wrapper for arch independent argument check */
 

src/fenv/m68k/fenv.c

@@ -1,5 +1,5 @@
 #include <fenv.h>
-#include "libc.h"
+#include <features.h>
 
 #if __HAVE_68881__ || __mcffpu__
 

src/fenv/powerpc64/fenv.c

@@ -1,6 +1,6 @@
 #define _GNU_SOURCE
 #include <fenv.h>
-#include "libc.h"
+#include <features.h>
 
 static inline double get_fpscr_f(void)
 {

src/fenv/s390x/fenv.c

@@ -1,5 +1,5 @@
 #include <fenv.h>
-#include "libc.h"
+#include <features.h>
 
 static inline unsigned get_fpc(void)
 {