systools: fix gcc version detecton on non-english locale

Summary:
ghc runs 'gcc -v' to check if we run under vanilla gcc
or disaguised clang by checking for string

    "gcc version <something>"

But this check does not always work as gcc has that string
localized via gettext mechanism:

    (some gcc's locale strings)
    be.po-msgstr "версія gcc %s\n"
    da.po-msgstr "GCC version %s\n"
    de.po-msgstr "gcc-Version %s %s\n"
    el.po-msgstr "έκδοση gcc %s\n"
    ...

To ping gcc to English locale we now override environment
variable with 'LANGUAGE=en' value.

Fixes Issue #8825

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>

Test Plan: validate

Reviewers: austin

Reviewed By: austin

Subscribers: simonmar, ezyang, carter

Differential Revision: https://phabricator.haskell.org/D185

GHC Trac Issues: #8825