[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):
See discussion from r334881:
@@ -113,7 +113,7 @@ TEST(SanitizerCommon, FileOps) {
fd = OpenFile(tmpfile, WrOnly);
ASSERT_NE(fd, kInvalidFd);
-#if SANITIZER_POSIX
+#if SANITIZER_POSIX && !SANITIZER_MAC
EXPECT_EQ(internal_lseek(fd, 0, SEEK_END), 0u);
#endif
uptr bytes_written = 0;
