[compiler-rt] Fix CompilerTargetTriple for non-English locales.

When the locale of a shell is set other than English locales or the C locale,
The word 'Target' may be translated. Thus, with e.g. ja_JP locale, compiler-rt
couldn't be built properly. Forcing LANG=C fixes the problem.

Patch by Ogino Masanori.

llvm-svn: 201506