The default locale can be changed in some environments with environment
variables. These environment variables used to allow the system to get
into an invalid state, where the default locale was unsupported. This
patch detects that case and falls back to "und" as the default locale
if there is an Intl service which does not support the locale that ICU
reports as the default. It also has a slight cleanup of surrounding code.

I haven't gone through the work to set up an automated test, as triggering
the case requires setting environment variables, which our tests don't tend
to do, but I tested interactively as follows:

dehrenberg@dehrenberg:~/v8/v8$ LC_ALL="tlh-FR" rlwrap out/Release/d8
V8 version 5.7.0 (candidate)
d8> new Intl.NumberFormat("foo").resolvedOptions().locale
"und"
d8> new Intl.NumberFormat().resolvedOptions().locale
"und"
d8>

dehrenberg@dehrenberg:~/v8/v8$ LC_ALL="de" rlwrap out/Release/d8
V8 version 5.7.0 (candidate)
d8> new Intl.NumberFormat().resolvedOptions().locale
"de"
d8> new Intl.NumberFormat("foo").resolvedOptions().locale
"de"
d8>

BUG=v8:4216

Review-Url: https://codereview.chromium.org/2646593002
Cr-Commit-Position: refs/heads/master@{#43253}