tools/nolibc: rename my_syscallX() to __nolibc_syscallX()

tools/nolibc: rename my_syscallX() to __nolibc_syscallX()

The naming convention of the my_syscallX() macros is a bit unfortunate.
They may conflict with application code and the name is very generic.

Switch to __nolibc_syscallX(). The leading underscores place the symbols
in the implementation-defined namespace, avoiding conflicting names.
It is also clearer that these are non-standard extensions from nolibc.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://patch.msgid.link/20260223-nolibc-namespacing-v1-1-52574ffebb2c@weissschuh.net