[msan] Fix ReExec on linux.

execle(argv[0], ...) is wrong, because argv[0] may contain a PATH-resolvable
program name. Execute /proc/self/exe instead.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@175504 91177308-0d34-0410-b5e6-96231b3b80d8