x86: Use operand size 4 when it would be 2 for cmpxchg8b.

This means the instruction is treated as cmpxchg8b when the effective
operand size is 16 bits.

Change-Id: I4d9bb295f96097e1746a9bbccb2c579d14738fab
Reviewed-on: https://gem5-review.googlesource.com/6603
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>

x86: revamp cmpxchg8b/cmpxchg16b implementation

The previous implementation did a pair of nested RMW operations,
which isn't compatible with the way that locked RMW operations are
implemented in the cache models. It was convenient though in that
it didn't require any new micro-ops, and supported cmpxchg16b using
64-bit memory ops. It also worked in AtomicSimpleCPU where
atomicity was guaranteed by the core and not by the memory system.
It did not work with timing CPU models though.

This new implementation defines new 'split' load and store micro-ops
which allow a single memory operation to use a pair of registers as
the source or destination, then uses a single ldsplit/stsplit RMW
pair to implement cmpxchg. This patch requires support for 128-bit
memory accesses in the ISA (added via a separate patch) to support
cmpxchg16b.

x86: Add microop for fence
This patch adds a new microop for memory barrier. The microop itself does
nothing, but since it is marked as a memory barrier, the O3 CPU should flush
all the pending loads and stores before the fence to the memory system.

copyright: Change HP copyright on x86 code to be more friendly

X86: Turn the CMPXCHG8B microcode into a template and generate each variant.

X86: Implement the CMPXCHG8B/CMPXCHG16B instruction.

X86: Implement a locking version of XADD.

X86: Implement a locking version of CMPXCHG.

X86: Implement the xadd instruction.

style: Remove non-leading tabs everywhere they shouldn't be. Developers should configure their editors to not insert tabs

X86: Put ldst into the microcode (the earlier changeset didn't really).
Also clean things up as much as possible so that faulting won't break an
instruction. More microops which verify addresses are needed.

X86: Put in stubs for x87, 64 bit and 128 bit SIMD instruction microcode.