* work around a stage1 miscompilation leading to the wrong integer
comparison predicate being emitted.
* fix the bug of not annotating callsites with the calling convention
of the callee, leading to undefined behavior.
* add the `nobuiltin` attribute when building freestanding libc or
compiler_rt libraries to prevent e.g. memcpy from being "optimized"
into a call to itself.
* compiler-rt: change a call to be comptime to make the generated LLVM
IR simpler and easier to study.
I still can't enable the widening tests due to the compiler-rt compare
function being miscompiled in some not-yet-diagnosed way.
9ed599b4e3