Teach LLVM about a PIE option which, when enabled on top of PIC, makes

Teach LLVM about a PIE option which, when enabled on top of PIC, makes
optimizations which are valid for position independent code being linked
into a single executable, but not for such code being linked into
a shared library.

I discussed the design of this with Eric Christopher, and the decision
was to support an optional bit rather than a completely separate
relocation model. Fundamentally, this is still PIC relocation, its just
that certain optimizations are only valid under a PIC relocation model
when the resulting code won't be in a shared library. The simplest path
to here is to expose a single bit option in the TargetOptions. If folks
have different/better designs, I'm all ears. =]

I've included the first optimization based upon this: changing TLS
models to the *Exec models when PIE is enabled. This is the LLVM
component of PR12380 and is all of the hard work.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@154294 91177308-0d34-0410-b5e6-96231b3b80d8