[AArch64] Merge globals when optimising for size

Extern global merging is good for code-size. There's definitely potential for
performance too, but there's one regression in a benchmark that needs
investigating, so that's why we enable it only when we optimise for size for
now.

Patch by Ramakota Reddy and Sjoerd Meijer.

Differential Revision: https://reviews.llvm.org/D61947

llvm-svn: 363130