tools/nolibc/stdlib: add i64toa() and u64toa()

These are 64-bit variants of the itoa() and utoa() functions. They also
support reentrant ones, and use the same itoa_buffer. The functions are
a bit larger than the previous ones in 32-bit mode (86 and 98 bytes on
x86_64 and armv7 respectively), which is why we continue to provide them
as separate functions.

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