Enable frozen_string_literal in builtin_iseq_load

[jhawthorn]

Currently this has a fairly minor effect as strings are not used heavily inside the builtins (outside of warnings, requires, and errors). Hopefully this allows us to use strings in the future where appropriate (maybe things like NilClass#to_s...).

I noticed this when looking at

$ ./ruby --disable-gems -e 'puts RubyVM::InstructionSequence.disasm(Time.instance_method(:initialize))'

which previously had

0043 putstring  "timezone argument given as positional and keyword arguments"

and on this branch has

0043 putobject  timezone argument given as positional and keyword arguments"