code_env: use feature test for PR_GET_NAME support

Function `get_process_name` has platform specific dependencies. Check
for Linux prctl function and correct command flag.

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
Merge pull request #607 from ceph/prctl-getname-test

code_env: use feature test for PR_GET_NAME support

Reviewed-by: Sage Weil <sage@inktank.com>