en_US_POSIX is ICU's fallback locale on POSIX platforms for when it
cannot detect a default locale.  Consider it equivalent to en-US.

I won't claim this is the best possible fix but it makes the following
tests pass again for me locally:

    intl/break-iterator/default-locale
    intl/break-iterator/wellformed-unsupported-locale
    intl/collator/default-locale
    intl/collator/wellformed-unsupported-locale
    intl/date-format/default-locale
    intl/date-format/wellformed-unsupported-locale
    intl/number-format/default-locale
    intl/number-format/wellformed-unsupported-locale

R=jgruber@chromium.org

Cq-Include-Trybots: master.tryserver.v8:v8_linux_noi18n_rel_ng
Change-Id: I0e8029f4ffaf5fd3ca72a1cc5db2878891744864
Reviewed-on: https://chromium-review.googlesource.com/567981
Commit-Queue: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46730}