Add test for compiling float literals in locales with comma decimal separators.

Handle inexplicable test setlocale failure on Android,Linux. (Require success on other platforms)

Skip setting the locale on Android, which is always C locale in C++, but for some
reason std::locale::classic isn't implemented as a no-op.

Bug: angleproject:2607
Test: angle_unittests
Change-Id: I7c97cb56c01335db46f532fb8af3f9a4f2a30564
Reviewed-on: https://chromium-review.googlesource.com/1077789Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>