SysV AMD64 ABI: don't rewrite to a single bit.

Otherwise std.uni cannot be compiled:
getAbiType(BitPacked!(bool, 1LU)) mismatch: i1 vs. i8
gen/tollvm.cpp:527: void DtoStore(llvm::Value*, llvm::Value*): Assertion `src->getType() != llvm::Type::getInt1Ty(gIR->context()) && "Should store bools as i8 instead of i1."' failed.