Fix namespace violation in stdio.h and sys/stat.h if build with optimization. [BZ #26376]

If build with optimization, stdio.h and sys/stat.h are defining some inlining
functions.  This leads to test fails if glibc is build with the following
commands. (Note that the conformtests usually builds without optimization or
other CFLAGS):
<glibc>/configure CC="gcc -O3" --prefix=/usr
make
make subdirs=conform check
- FAIL: conform/XPG4/stdio.h/conform
- FAIL: conform/XPG42/stdio.h/conform
out-files:
...
PASSCOMBINED: Availability of variable optopt
PASSCOMBINED: Type of variable optopt
    Namespace violation: "getc_unlocked"
    Namespace violation: "getchar_unlocked"
    Namespace violation: "putc_unlocked"
    Namespace violation: "putchar_unlocked"
FAIL: Namespace of <stdio.h>
----------------------------------------------------------------------------
  Total number of tests   :  168
  Number of failed tests  :    1
  Number of xfailed tests :    0
  Number of skipped tests :    0

- FAIL: conform/POSIX2008/sys/stat.h/conform
out-file:
...
PASSCOMBINED: Availability of function utimensat
PASSCOMBINED: Type of function utimensat
    Namespace violation: "mknodat"
FAIL: Namespace of <sys/stat.h>
----------------------------------------------------------------------------
  Total number of tests   :   97
  Number of failed tests  :    1
  Number of xfailed tests :    0
  Number of skipped tests :    0

For getc_unlocked, getchar_unlocked, putc_unlocked, putchar_unlocked in stdio.h,
those are defined "# ifdef __USE_POSIX" instead of "#ifdef __USE_POSIX199506"
for the non-inlining declaration. See also
"Bug 20014 - stdio.h namespace for pre-threads POSIX"
(https://sourceware.org/bugzilla/show_bug.cgi?id=20014).

For mknodat in sys/stat.h, those are defined "# ifdef __USE_ATFILE" instead of
the additional guard "# if defined __USE_MISC || defined __USE_XOPEN_EXTENDED".

io/sys/stat.h

@@ -486,13 +486,15 @@ __NTH (mknod (const char *__path, __mode_t __mode, __dev_t __dev))
 }
 # endif
 
-# ifdef __USE_ATFILE
+# if defined __USE_MISC || defined __USE_XOPEN_EXTENDED
+#  ifdef __USE_ATFILE
 __extern_inline int
 __NTH (mknodat (int __fd, const char *__path, __mode_t __mode,
 		__dev_t __dev))
 {
   return __xmknodat (_MKNOD_VER, __fd, __path, __mode, &__dev);
 }
+#  endif
 # endif
 
 # if defined __USE_LARGEFILE64 \

libio/bits/stdio.h

@@ -60,7 +60,7 @@ fgetc_unlocked (FILE *__fp)
 # endif /* misc */
 
 
-# ifdef __USE_POSIX
+# ifdef __USE_POSIX199506
 /* This is defined in POSIX.1:1996.  */
 __STDIO_INLINE int
 getc_unlocked (FILE *__fp)
@@ -95,7 +95,7 @@ fputc_unlocked (int __c, FILE *__stream)
 # endif /* misc */
 
 
-# ifdef __USE_POSIX
+# ifdef __USE_POSIX199506
 /* This is defined in POSIX.1:1996.  */
 __STDIO_INLINE int
 putc_unlocked (int __c, FILE *__stream)