Connected sanitizer libraries to the build (for x86)

[Dimitry Andric]

Hi,

In r277146, I have connected the sanitizer libraries from compiler-rt to
the build.  Currently, this works for i386 and amd64, and contains
Address Sanitizer (ASan) and Undefined Behavior Sanitizer.

AddressSanitizer is a fast memory error detector [1].  It consists of a
compiler instrumentation module and a run-time library. The tool can
detect the following types of bugs:

* Out-of-bounds accesses to heap, stack and globals
* Use-after-free
* Use-after-return (to some extent)
* Double-free, invalid free
* Memory leaks (experimental)

The typical slowdown introduced by AddressSanitizer is 2x.  Enable it by
compiling and linking with clang, and using the -fsanitize=address flag.

Undefined Behavior Sanitizer is a fast and compatible undefined behavior
checker, which enables a number of checks that have small runtime cost
and no impact on address space layout or ABI.  Enable it by using the
-fsanitize=undefined flag. [2]

Please note that the sanitizers still have some rough edges on FreeBSD,
particularly on i386.  These will hopefully be smoothed out in the
coming time.  Reports of problems (and fixes :) are very welcome, but
please log them in Bugzilla, so they can be tracked.

-Dimitry

[1] http://llvm.org/releases/3.5.0/tools/clang/docs/AddressSanitizer.html
[2] http://llvm.org/releases/3.5.0/tools/clang/docs/UsersManual.html#opt-fsanitize-undefined

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