Add support for positionally-encoded operands to FixedLenDecoderEmitter

Unfortunately, the PowerPC instruction definitions make heavy use of the
positional operand encoding heuristic to map operands onto bitfield variables
in the instruction definitions. Changing this to use name-based mapping is not
trivial, however, because additional infrastructure needs to be designed to
handle mapping of complex operands (with multiple suboperands) onto multiple
bitfield variables.

In the mean time, this adds support for positionally encoded operands to
FixedLenDecoderEmitter, so that we can generate a disassembler for the PowerPC
backend. To prevent an accidental reliance on this feature, and to prevent an
undesirable interaction with existing disassemblers, a backend must opt-in to
this support by setting the new decodePositionallyEncodedOperands
instruction-set bit to true.

When enabled, this iterates the variables that contribute to the instruction
encoding, just as the encoder does, and emulates the procedure the encoder uses
to map "numbered" operands to variables. The bit range for each variable is
also determined as the encoder determines them. This map is then consulted
during the decoder-generator's loop over operands to decode, allowing the
decoder to understand both position-based and name-based operand-to-variable
mappings.

As noted in the comment on the decodePositionallyEncodedOperands definition,
this support should be removed once it is no longer needed. There should be no
change to existing disassemblers.
Support for microMIPS LL and SC instructions.
Add a disassembler to the PowerPC backend

The tests for the disassembler were adapted from the encoder tests, and for the
most part, the output from the disassembler matches that encoder-test inputs.
There are some places where more-informative mnemonics could be produced
(notably for the branch instructions), and those cases are noted in the tests
with FIXMEs.

Future work includes:

 - Generating more-informative mnemonics when possible (this may also be done
   in the printer).

 - Remove the dependence on positional "numbered" operand-to-variable mapping
   (for both encoding and decoding).

 - Internally using 64-bit instruction variants in 64-bit mode (if this turns
   out to matter).
Long doubles are required to be aligned to 128 bits and svr4 32 bits.

Clang was already getting this right.
Fix comment typo.
Support for microMIPS control instructions.