[test] Use system locale for mri-utf8.test

Summary:
llvm-ar's mri-utf8.test test relies on the en_US.UTF-8 locale to be
installed for its last RUN line to work. If not installed, the unicode
string gets encoded (interpreted) as ascii which fails since the most
significant byte is non zero. This commit changes the test to only rely
on the system being able to encode the pound sign in its default
encoding (e.g. UTF-16 for Microsoft Windows) by always opening the file
via input/output redirection. This avoids forcing a given locale to be
present and supported. A Byte Order Mark is also added to help
recognizing the encoding of the file and its endianness. Finally the
XFAIL on system-darwin is removed since the test actually passes fine on
Mac OS X and XFAIL was only added because it failed before.

Reviewers: gbreynoo, MaskRay, rupprecht, JamesNagurne, jfb

Subscribers: dexonsmith, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68472