Previously it would use a bitcast, which cannot directly be used on AVR
because functions live in a different address space on AVR. To fix this,
use a ptrtoint/inttoptr pair.
This allows testdata/coroutines.go to be compiled, but due to what
appears to be an LLVM bug cannot be optimized and codegen'ed:
tinygo: /home/ayke/src/github.com/tinygo-org/tinygo/llvm-project/llvm/lib/IR/Constants.cpp:1776: static llvm::Constant *llvm::ConstantExpr::getBitCast(llvm::Constant *, llvm::Type *, bool): Assertion `CastInst::castIsValid(Instruction::BitCast, C, DstTy) && "Invalid constantexpr bitcast!"' failed.
This happens as one of the function passes after the TinyGo passes and
after the module has been verified so most likely it is a bug somewhere
in LLVM.
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