systemd: change default locale from C.UTF-8 to C

systemd: change default locale from C.UTF-8 to C

current default locale is set to C.UTF-8, but glibc not support
locale C.UTF-8. so set to the default locale C.

[snip]
        if not meson.is_cross_build()
                choose_default_locale_sh = find_program('tools/choose-default-locale.sh')
                default_locale = run_command(choose_default_locale_sh).stdout().strip()
        else
                default_locale = 'C.UTF-8'
        endif

if default locale set to C.UTF-8, it will cause libpcre ptest fail:

  re> //8+L
** Failed to set locale ""

Signed-off-by: Changqing Li <changqing.li@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>