Incomplete semantics for mrc instruction

[RyanGlScott]

AArch32 defines an mrc instruction (Move Register to Coprocessor) which has many different uses, depending on which coprocessor is specified. Among other things, mrc can be used to access thread-local state. For a more complete writeup on how mrc can be used, refer to a reference manual for a specific ARM CPU (e.g., for Cortex-A7).

Unfortunately, asl-translator's semantics for the mrc instruction (Move Register to Coprocessor) are incomplete. This is because mrc is defined in terms of the AArch32.SysRegRead primitive, but that is simply defined as:

asl-translator/data/support.asl

Lines 644 to 646 in f4a89de

	bits(32) AArch32.SysRegRead(integer cp_num, bits(32) instr)
	assert FALSE;
	return Zeros(32);

This simply returns zeroes, which is almost certainly not going to behave like one would expect it to.

Defining the full semantics for mrc will likely be a lot of work, as there are many different ARM coprocessor flavors out there. We may want to add support for a single coprocessor at a time as the need arises.