Make sure to update Target arch if environment changed

Summary: Fixes "target list" for non-android linux platforms (ie gnu, gnueabi)

Reviewers: jasonmolenda, tfiala, clayborg, tberghammer

Subscribers: tberghammer, danalbert, lldb-commits, sas

Differential Revision: http://reviews.llvm.org/D18631

Change by Francis Ricci <fjricci@fb.com>

This is a cherry-pick of r265419

llvm-svn: 270126