bug#39248: format-time-string ignores user's preferred locale

[Paul W. Rankin]

Calling function (format-time-string "%x") will output incorrect time
string, ignoring user's preferred locale.

* Steps to reproduce:

1. ensure locale is correct with M-x getenv RET LANG RET
    -> en_AU.UTF-8
2. M-: (format-time-string "%x") RET
    -> "01/23/20"
3. repeat for env LC_TIME

* Expected results:

The format for %x as per the docs:
    %x is the locale’s "preferred" date format.

en_AU locale's "preferred" date format should be DD/MM/YYYY:
"23/01/2020"

i.e. the same as output from shell:
    $ locale
    LANG="en_AU.UTF-8"
    LC_COLLATE="en_AU.UTF-8"
    LC_CTYPE="en_AU.UTF-8"
    LC_MESSAGES="en_AU.UTF-8"
    LC_MONETARY="en_AU.UTF-8"
    LC_NUMERIC="en_AU.UTF-8"
    LC_TIME="en_AU.UTF-8"
    LC_ALL=
    $ date +%x
    23/01/2020

* Actual results:

"01/23/20"

GNU Emacs 27.0.60 (build 1, x86_64-apple-darwin19.2.0, NS appkit-1894.20
Version 10.15.2 (Build 19C57)) of 2020-01-18