Compiling yklua without -O2 fails

[ltratt]

At the moment yklua hardcodes -O2 (should it? dunno!). If you remove O2 (edit src/Makefile and set OPTFLAGS= i.e. to the empty string) and then try and compile yklua we get a trace_builder panic:

thread '<unnamed>' panicked at ykrt/src/compile/jitc_yk/trace_builder.rs:266:22:
not yet implemented: Unimplemented { tyidx: TyIdx(6), llvm_inst_str: "  call void
 @llvm.memcpy.p0.p0.i64(ptr align 8 %1915, ptr align 8 %1917, i64 8, i1 false)" }