Add LTO support and corresponding documentation

These two commits add support to compile libsoftboundcets_rt with as a bitcode library. This means it can then be used together with -flto, and the calls to the check functions can be inlined.

For some benchmarks, this makes a large difference. Also, all softbound papers seem to assume that checks are inlined. Here is a quick comparison using SPEC:

Tool	| SoftBound Vanilla | SoftBound LTO
------|------------------------|--------------------
milc | 45.3 | 29.7
lbm | 14.1 | 14.0
bzip2 | 25.3 | 24.1
sjeng | 15.0 | 11.9
h264ref | 143 | 80.6

Hope this helps!
-- Jonas