[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

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@334944 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed