[sanitizer] Use AT_EXECFN in ReExec() if available

execve("/proc/self/exe") will not work if the binary relies on
$EXEC_ORIGIN in an rpath. Query AT_EXECFN instead, which will give the
same string that the current binary was exec'd with.

Differential Revision: https://reviews.llvm.org/D54113

llvm-svn: 346215