S2E ARM port (linearized history)

This is a complete port of S2E to ARM targets based on the original arm-experimental branch.
ARM is now a modular target like i386 and x86_64, and support both 16 bits (Thumb) and 32 bits (ARM) opcodes in little-endian mode.

We chose to diverge from the original experimental port for the following choices:
 * added the ARM target side-by-side with existing x86 targets
 * use CPUArchState over target-specific ones
 * conditionally build-protect minimal regions of target-specific code

This branch also contains:
 * five commits backport from qemu.git, needed for arm-specific bugfixes
 * two host-specific hacks to workaround compilers faults experienced by Jonas

A still-growing acceptance testsuite is currently available at https://github.com/eurecom-s3/s2e-arm-testsuite.