benchmark/bm_hash_aref_sym*.rb: force static symbols

Dynamic symbols hash more slowly because they need extra method
dispatch in rb_any_hash.  I am not sure if dynamic symbols are
a realistic use case as hash keys, so this commit only
restores performance when comparing against versions of Ruby
which lack dsyms.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47854 b2dd03c8-39d4-4d8f-98ff-823fe69b080e