[sanitizer] Support running without fd 0,1,2.

Summary:
Support running with no open file descriptors (as may happen to
"init" process on linux).
* Remove a check that writing to stderr succeeds.
* When opening a file (ex. for log_path option), dup the new fd out of
[0, 2] range to avoid confusing the program.

(2nd attempt, this time without the sanitizer_rtems change)

Reviewers: pcc, vitalybuka

Subscribers: kubamracek, llvm-commits

Differential Revision: https://reviews.llvm.org/D55801

llvm-svn: 349817