Force the OS X version to 10.6 for old-style simulator builds.

The hack of recognizing a -D__IPHONE_OS_VERSION_MIN_REQUIRED option
in place of -mios-simulator-version-min leaves the Darwin version
unspecified.  It can be set separately with -mmacosx-version-min (which
makes no sense) or inferred to match the host version (which is unpredictable
and usually wrong).  This really needs to get cleaned up, but in the
meantime, force the OS X version to 10.6 so that the behavior is sane for
the iOS simulator.  Thanks for Argyrios for the patch.
<rdar://problem/11858187>

llvm-svn: 160484