lib: libc: Drop `z_` prefix from stdio syscalls

This commit removes the `z_` prefix from the stdio syscall functions
(`z_zephyr_write_stdout` and `z_zephyr_read_stdin`) since it is
redundant and does not align with the convention used by the equivalent
minimal libc syscall functions (e.g. `zephyr_fputc` and
`zephyr_fwrite`).

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>
3 files changed