instead of FILE*.
Using C.fprintf is slower than String manipulation on memory. I'm going
to change the way MJIT writes files, and this is a prerequisite for it.
except for bit fields.
I made a risky assumption on leading bit fields and just gave up
non-leading bit fields for now. I'll change it to let C code access bit
fields later.
I'm planning to introduce mjit_compiler.rb, and I want to make this
consistent with it. Consistency with compile.c doesn't seem important
for MJIT anyway.