Materialize global addresses via movt/movw pair, this is always better

Materialize global addresses via movt/movw pair, this is always better
than doing the same via constpool:
1. Load from constpool costs 3 cycles on A9, movt/movw pair - just 2.
2. Load from constpool might stall up to 300 cycles due to cache miss.
3. Movt/movw does not use load/store unit.
4. Less constpool entries => better compiler performance.

This is only enabled on ELF systems, since darwin does not have needed
relocations (yet).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@89720 91177308-0d34-0410-b5e6-96231b3b80d8