Fix loop unroll pragma for GCC < 8
Fix loop unroll pragma for GCC < 8

The 'GCC unroll' pragma was only introduced in GCC 8. Add appropriate
preprocessor checks to avoid warnings on earlier gcc versions.

Change-Id: I0278aba3a7e319a93cdefccc27b63a9e5450a12c
Reviewed-on: http://gerrit.cloudera.org:8080/15558
Tested-by: Todd Lipcon <2c763d726a8c2f6afda9be7e0e56a2caa6bb5e6b@apache.org>
Reviewed-by: Adar Dembo <866ca730bfc7f544e3ebce604ceeba4879f23df0@cloudera.com>

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