[sanitizer] Implement logging to syslog.

Previously, Android target had a logic of duplicating all sanitizer
output to logcat. This change extends it to all posix platforms via
the use of syslog, controlled by log_to_syslog flag. Enabled by
default on Android, off everywhere else.

A bit of cmake magic is required to allow Printf() to call a libc
function. I'm adding a stub implementation to support no-libc builds
like dfsan and safestack.

This is a second attempt. I believe I've fixed all the issues that
prompted the revert: Mac build, and all kinds of non-CMake builds
(there are 3 of those).

llvm-svn: 243051