Remove workarounds for panics

[bjorn3]

• (Panic with SimpleJIT + module.define_data bytecodealliance/cranelift#407) https://github.com/bjorn3/rustc_codegen_cranelift/blob/master/src/constant.rs#L82-L86
• (thread 'main' panicked at 'attempt to add with overflow' bytecodealliance/cranelift#432) https://github.com/bjorn3/rustc_codegen_cranelift/blob/master/src/base.rs#L171-L183 (b11cb57)
• (Simplejit gives a bus error bytecodealliance/cranelift#371) https://github.com/bjorn3/rustc_codegen_cranelift/blob/master/src/lib.rs#L268-L270
• https://github.com/bjorn3/rustc_codegen_cranelift/blob/master/src/base.rs#L52-L62