[llvm-objcopy] Add e_machine validity check for reserved section indexes

As discussed on llvm-commits it was decided it would be best to check
e_machine before declaring that a reserved section index is valid. The
only special e_machine value that matters here is EM_HEXAGON. This
change adds a special check for EM_HEXAGON.

Patch by Jake Ehrlich

Differential Revision: https://reviews.llvm.org/D37767

llvm-svn: 313114