[ELF] Add a NOP instruction to make LLVM generate a mapping symbol

This is a test change applicable for ARM targets.

Previously LLVM would generate a mapping symbol when code contains only
data. This was changed as part of

https://reviews.llvm.org/D30724

and to be more close to the ARM ELF ABI.

The test case is changed to check for behavior with minimal changes.

Patch by Shankar Easwaran

Differential Revision: https://reviews.llvm.org/D31500

llvm-svn: 299212