elf: Remove allocate use on _dl_debug_printf

The maximum number of directives is already limited by the maximum
value of iovec, and current padding usage on _dl_map_object_from_fd
specifies a value of 16 (2 times sizeof (void *)) in hexa, which is
less than the INT_STRLEN_BOUND(void *) (20 for LP64).

This works if pointers are larger than 8 bytes, for instance 16.
In this case the maximum padding would be 32 and the IFMTSIZE would
be 40.

The resulting code does use a slightly larger static stack, the
output of -fstack-usage (for x86_64):

 * master:
   dl-printf.c:35:1:_dl_debug_vdprintf     1344    dynamic

 * patch:
   dl-printf.c:36:1:_dl_debug_vdprintf     2416    static

However, there is an improvement in code generation:

 * master
   text    data     bss     dec     hex filename
   3309       0       0    3309     ced elf/dl-printf.os

 * patch
   text    data     bss     dec     hex filename
   3151       0       0    3151     c4f elf/dl-printf.os

Checked on x86_64-linux-gnu.

Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>