[ARM] Make GlobalMerge merge extern globals by default

Enabling merging of extern globals appears to be generally either beneficial or
harmless. On some benchmarks suites (on Cortex-M4F, Cortex-A9, and Cortex-A57)
it gives improvements in the 1-5% range, but in the rest the overall effect is
zero.

Differential Revision: http://reviews.llvm.org/D10966

llvm-svn: 243874