Support specifying the assembly that should be generated

We want to make sure that native code generator optimizations, like primop unrolling, always fire. We can do this by allowing the programmer to specify what assembly should be generated for a particular snippet of Cmm. The programmer gives a snippet of Cmm:

#include "Cmm.h"

// Large memcpy's should lower to calls.
callMemcpy
{
  W_ dst, src;
  prim %memcpy(dst "ptr", src "ptr", 1024, 4) [];
}

and the expected assembly output:

callMemcpy:
movq  ; Move arguments into place
movq
movl
movl
call memcpy

The expected output shouldn't mention specific register names as these are likely to change.

Trac metadata

Trac field	Value
Version	7.0.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Test Suite
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