Ultra-compatible memory mapping about memory-sanitizer HOT 5 OPEN

That's Linux-x86_64 of course.
Heap goes to 0x7d..0x7e, as in TSan.

There is a weird code size (and maybe performance) regression due to llvm doing 
something inefficient with xor(xor(x)) on X86:
http://llvm.org/bugs/show_bug.cgi?id=21749

We will probably win more by disabling PIE, so I'm inclined to go ahead with 
this anyway and optimize it later. This problem should be fixable with some 
kind of peephole optimization in the backend.

W/o disabling PIE performance measurements are inconclusive.
470.lbm speeds up by ~10% with the new mapping for some reason.
The rest are +-3%.
Code size grows by ~2.5%, but on one of the specs - by close to 10%.

Here is another idea with all the same properties.

#define MEM_TO_SHADOW(mem) (((uptr)(mem)) ^ 0x500000000000ULL)
#define SHADOW_TO_ORIGIN(mem) (((uptr)(mem)) + 0x100000000000ULL)

Here, shadow-to-origin is an addition of a constant, which can be wrapped into 
the addressing mode without using an extra register.

Memory layout:

000000000000 - 050000000000 app-1
050000000000 - 100000000000 shadow-2
100000000000 - 150000000000 invalid
150000000000 - 200000000000 origin-2
200000000000 - 300000000000 shadow-3
300000000000 - 400000000000 origin-3
400000000000 - 500000000000 invalid
500000000000 - 550000000000 shadow-1
550000000000 - 600000000000 app-2
600000000000 - 650000000000 origin-1
650000000000 - 700000000000 invalid
700000000000 - 800000000000 app-3

Performance is the same or better (old, new, new/old ratio):
       400.perlbench,      2822.00,      2849.00,         1.01
           401.bzip2,      1975.00,      1978.00,         1.00
             429.mcf,      1665.00,      1619.00,         0.97
           445.gobmk,      2257.00,      2224.00,         0.99
           456.hmmer,      2954.00,      2977.00,         1.01
           458.sjeng,      3613.00,      3567.00,         0.99
      462.libquantum,      1381.00,      1380.00,         1.00
         464.h264ref,      4348.00,      4366.00,         1.00
           473.astar,      1312.00,      1311.00,         1.00
       483.xalancbmk,      1287.00,      1227.00,         0.95
            433.milc,      1404.00,      1405.00,         1.00
            444.namd,      1684.00,      1690.00,         1.00
          447.dealII,      1309.00,      1305.00,         1.00
          450.soplex,       996.00,       983.00,         0.99
          453.povray,      1505.00,      1519.00,         1.01
             470.lbm,      1325.00,      1329.00,         1.00
         482.sphinx3,      2593.00,      2590.00,         1.00

Code size:
- instrumented code is smaller by 0.5% on average
- run-time library is larger by 40Kb, mostly because MEM_IS_APP(x) check is 
more complex, and it is inline in multiple places (ex. all interceptors, as 
part of __msan_poison()). A lot of these are redundant and can be removed later.

attaching the current patchset just in case