Increase the buffer size again, for testing hello_world.pexe

Reserving address space like this means we don't need to implement
generalised relocations yet.
Mark some instructions as unhandled and generate stub code that aborts
Allow unhandled ConstantExprs to generate stub code that aborts
Add stub for ConstantFP
Handle UndefValue
Add stub for i64 values in phi nodes
Generate stub code for floating point comparisons

We do this by matching ICmpInst rather than CmpInst since we only handle
integer comparisons.
Add stubs for arithmetic, loads and stores on doubles (FP)
Change unhandled_case() to generate stub code that aborts with a helpful message
Extend the stub for intrinsics to print the intrinsic name
Handle llvm.lifetime.start/end intrinsics by ignoring them
Add stubs for mandatory IRT interfaces; implement tls_init() and tls_get()
Implement the IRT sysbrk() interface for testing
Handle memset intrinsics
Handle memmove intrinsics
Generate stub for functions that are declared but not defined, e.g. llvm.nacl.read.tp
Implement the llvm.nacl.read.tp intrinsic

Move the TLS functions into runtime_helpers.c.
Handle ConstantExprs by expanding them out into instructions
Add option to generate log messages for tracing the code's execution
Run LLVM's IR verifier as a sanity check for our IR transforms

This catches a problem where instructions are placed before phi nodes
within a basic block when expanding out ConstantExprs.