[debugserver] Fix OSX build for older XCode versions after r251091.

This fixes the OSX build for XCode versions older than 7 by skipping
references to LC_VERSION_MIN_TVOS and LC_VERSION_MIN_WATCHOS if
TARGET_OS_TV or TARGET_OS_WATCH aren't defined.

Reviewed by: jasonmolenda
Subscribers: lldb-commits
Differential Revision: http://reviews.llvm.org/D14036

llvm-svn: 251172