[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.

llvm-svn: 242975