Have ReExec read environment from /proc/self/environ

It is possible for ReExec to be called before the C standard library
has initialised itself sufficiently for execv to work properly.
Fix this by parsing the environment ourselves and using execve.

Differential Revision: http://llvm-reviews.chandlerc.com/D304

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@172748 91177308-0d34-0410-b5e6-96231b3b80d8