[Hexagon] Fix compilation error with GCC 6

Compiling Hexagon target with GCC 6 produces "error: should have been
declared inside" due to GCC PR c++/69657 which was merged.

Properly wrapping operator<<() definitions within the namespace llvm
fixes the issue.

Author: domagoj.stolfa

Differential Revision: http://reviews.llvm.org/D17281

llvm-svn: 261220