Update LLVM and jettison jit support

LLVM's JIT has been updated numerous times, and we haven't been tracking it at
all. The existing LLVM glue code no longer compiles, and the JIT isn't used for
anything currently.

This also rebases out the FixedStackSegment support which we have added to LLVM.
None of this is still in use by the compiler, and there's no need to keep this
functionality around inside of LLVM.

This is needed to unblock #10708 (where we're tripping an LLVM assertion).

src/rustllvm/PassWrapper.cpp

@@ -81,7 +81,6 @@ LLVMRustCreateTargetMachine(const char *triple,
     TargetOptions Options;
     Options.NoFramePointerElim = true;
     Options.EnableSegmentedStacks = EnableSegmentedStacks;
-    Options.FixedStackSegmentSize = 2 * 1024 * 1024; // XXX: This is too big.
     Options.FloatABIType =
          (Trip.getEnvironment() == Triple::GNUEABIHF) ? FloatABI::Hard :
                                                         FloatABI::Default;