selftests/nolibc: make functions static if possible

This allows the compiler to generate warnings if they go unused.

Functions that are supposed to be used as breakpoints should not be
static, so un-statify those if necessary.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Willy Tarreau <w@1wt.eu>