This allows me to begin enabling (or backing out) misched by default
for one subtarget at a time. To run misched we typically want to:
- Disable SelectionDAG scheduling (use the source order scheduler)
- Enable more aggressive coalescing (until we decide to always run the coalescer this way)
- Enable MachineScheduler pass itself.

Disabling PostRA sched may follow for some subtargets.

llvm-svn: 167826

llvm-svn: 134259

llvm-svn: 88682

llvm-svn: 86634

llvm-svn: 45418

llvm-svn: 22523

llvm-svn: 22394

Implement the X86 Subtarget.

This consolidates the checks for target triple, and setting options based
on target triple into one place.  This allows us to convert the asm printer
and isel over from being littered with "forDarwin", "forCygwin", etc. into
just having the appropriate flags for each subtarget feature controlling
the code for that feature.

This patch also implements indirect external and weak references in the
X86 pattern isel, for darwin.  Next up is to convert over the asm printers
to use this new interface.

llvm-svn: 22389