Add -fno-movt frontend option, to disable movt/movw on ARM

Summary:
In rL256641, @davide turned off movt generation by default for FreeBSD.
This was because our ld is very old, and did not support the relocations
for it.  However, Ian Lepore added the support very recently, so we
would like to revert rL256641, and replace it with a new `-fno-movt`
frontend option.  This way, it can be turned off when needed.

Reviewers: dexonsmith, echristo, emaste, davide

Subscribers: andrew, aemerson, rengolin, davide, cfe-commits, ahatanak, emaste

Differential Revision: http://reviews.llvm.org/D15899

llvm-svn: 256920