Add some comments to new frame entries


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174515 91177308-0d34-0410-b5e6-96231b3b80d8
Implement external weak (ELF) symbols on AArch64

Weakly defined symbols should evaluate to 0 if they're undefined at
link-time. This is impossible to do with the usual address generation
patterns, so we should use a literal pool entry to materlialise the
address.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174518 91177308-0d34-0410-b5e6-96231b3b80d8