tools/nolibc/stdlib: replace the ltoa() function with more efficient ones

The original ltoa() function and the reentrant one ltoa_r() present a
number of drawbacks. The divide by 10 generates calls to external code
from libgcc_s, and the number does not necessarily start at the beginning
of the buffer.

Let's rewrite these functions so that they do not involve a divide and
only use loops on powers of 10, and implement both signed and unsigned
variants, always starting from the buffer's first character. Instead of
using a static buffer for each function, we're now using a common one.

In order to avoid confusion with the ltoa() name, the new functions are
called itoa_r() and utoa_r() to distinguish the signed and unsigned
versions, and for convenience for their callers, these functions now
reutrn the number of characters emitted. The ltoa_r() function is just
an inline mapping to the signed one and which returns the buffer.

The functions are quite small (86 bytes on x86_64, 68 on armv7) and
do not depend anymore on external code.

Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>