stage2: LLVM backend: miscompilation fixes

 * 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.

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