[sanitizer] Get full argv and envp on glibc.

/proc/$PID/cmdline is clipped to 4Kb.
Locate argv and envp on the main thread stack.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@175163 91177308-0d34-0410-b5e6-96231b3b80d8