[hexagon] restore -fuse-cxa-atexit by default

"-fno-use-cxa-atexit" was a default provided by the initial
commit offering hexagon support.  This is no longer required.

Reviewers: bcahoon, sidneym

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D50816

llvm-svn: 339979