tools/nolibc/stdio: add a minimal [vf]printf() implementation

This adds a minimal vfprintf() implementation as well as the commonly
used fprintf() and printf() that rely on it.

For now the function supports:
  - formats: %s, %c, %u, %d, %x
  - modifiers: %l and %ll
  - unknown chars are considered as modifiers and are ignored

It is designed to remain minimalist, despite this printf() is 549 bytes
on x86_64. It would be wise not to add too many formats.

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