tools/nolibc: rename sys_foo() functions to _sys_foo()

tools/nolibc: rename sys_foo() functions to _sys_foo()

The sys_foo() naming scheme used by the syscall wrappers may collide
with application symbols. Especially as 'sys_' is an obvious naming
scheme an application may choose for its own custom systemcall wrappers.

Avoid these conflicts by using an leading underscore which moves the
names into the implementation's namespace. This naming scheme was chosen
over a '__nolibc_' prefix, as these functions are not an implementation
detail but a documented interface meant to be used by applications.

While this may break some existing users, adapting them should be
straightforward. Given that nolibc is most-likely vendored, no
unexpected breakage should happen. No in-tree users are affected.

These conflicts happen when compiling some of the kernel selftests
with nolibc.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://patch.msgid.link/20260319-nolibc-namespacing-v1-1-33c22eaddb5e@weissschuh.net