[PATCH] Enable FastISel on ARM for Linux and NaCl, not MCJIT

[JF Bastien]

This is a resubmit of r182877, which was reverted because it broken MCJIT
tests on ARM. The patch leaves MCJIT on ARM as it was before: only enabled
for iOS. I've CC'ed people from the original review and revert.

FastISel was only enabled for iOS ARM and Thumb2, this patch enables it for
ARM (not Thumb2) on Linux and NaCl, but not MCJIT.

Thumb2 support needs a bit more work, mainly around register class
restrictions.

The patch punts to SelectionDAG when doing TLS relocation on non-Darwin
targets. I will fix this and other FastISel-to-SelectionDAG failures in a
separate patch.

The patch also forces FastISel to retain frame pointers: iOS always keeps
them for backtracking (so emitted code won't change because of this), but
Linux was getting much worse code that was incorrect when using big frames
(such as test-suite's lencod). I'll also fix this in a later patch, it will
probably require a peephole so that FastISel doesn't rematerialize frame
pointers back-to-back.

The test changes are straightforward, similar to:

http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20130513/174279.html
They also add a vararg test that got dropped in that change.

I ran all of lnt test-suite on A15 hardware with --optimize-option=-O0 and
all the tests pass. All the tests also pass on x86 make check-all. I also
re-ran the check-all tests that failed on ARM, and they all seem to pass.
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