build system: -fno-builtin-printf

build system: -fno-builtin-printf

Benefits are: drops reference to out-of-line putchar(), fixes a few cases
of failed string merge.

function                                             old     new   delta
i2cdump_main                                        1488    1502     +14
sha256_process_block64                               423     433     +10
sendmail_main                                       1183    1185      +2
list_table                                          1114    1116      +2
i2cdetect_main                                      1235    1237      +2
fdisk_main                                          2852    2854      +2
builtin_type                                         119     121      +2
unicode_conv_to_printable2                           325     324      -1
scan_recursive                                       380     378      -2
mkfs_minix_main                                     2687    2684      -3
buffer_fill_and_print                                178     169      -9
putchar                                              152       -    -152
------------------------------------------------------------------------------
(add/remove: 0/2 grow/shrink: 7/4 up/down: 34/-167)          Total: -133 bytes
   text    data     bss     dec     hex filename
 937788     932   17676  956396   e97ec busybox_old
 937564     932   17676  956172   e970c busybox_unstripped

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>