Fix SO entry is main executable detection on android

* On Android (at least on platfrom-21 x86) the dynamic linker reports the
  executable with its full path
* Use the platform path of the executable when storing it into the cache
  (used to identify the SO entry for the executable) as this is the path
  what will be reported by the dynamic linker. If the platform path isn't
  set (local debugging) then it falls back to the normal file path.

Differential revision: http://reviews.llvm.org/D8296

llvm-svn: 232158