[sanitizer] Guard call to internal_lseek when SANITIZER_MAC is true

r334881 breaks macOS bots because internal_lseek is not defined (neither
used on macOS):
http://green.lab.llvm.org/green/job/clang-stage1-configure-RA/46240/consoleFull.

See discussion from r334881: https://reviews.llvm.org/rL334881

llvm-svn: 334944