libc/stdio: Rework custom streams interface similar to glibc.

libc/stdio: Rework custom streams interface similar to glibc.

Save 20 bytes per FILE structure, avoid indirect call for
read/write/seek/close operations for normal streams.
Additionally, custom streams has fileno = -2 now, like in glibc.

bloat-o-meter report (UCLIBC_HAS_GLIBC_CUSTOM_STREAMS=y):
function                                             old     new   delta
fopencookie                                           69     131     +62
ftello64                                             233     260     +27
fseeko64                                             298     319     +21
fclose                                               423     442     +19
.rodata                                            16696   16708     +12
fileno_unlocked                                       53      45      -8
__ns_name_pack                                       859     851      -8
vswscanf                                             184     144     -40
vdprintf                                             231     187     -44
vsscanf                                              210     151     -59
vswprintf                                            269     201     -68
vsnprintf                                            249     181     -68
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 5/7 up/down: 141/-295)         Total: -154 bytes

Signed-off-by: Leonid Lisovskiy <lly.dev@gmail.com>
Signed-off-by: Waldemar Brodkorb <wbx@uclibc-ng.org>