reboot.2: 1-argument reboot() is also provided by alternative libc

reboot.2: 1-argument reboot() is also provided by alternative libc

As far as the patch author knows, at least the following
alternative C libraries for linux also provide the
1-argument variant:

* uclibc: all public releases.
* dietlibc: 0.7.2 and all later releases; reboot() was not provided in
            0.7.1 and earlier releases.
* musl: all public releases.
* klibc: all public releases.
* bionic: all public releases.

... which should already cover most use cases.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>