MC: handle ParseDirective properly

Target specific parsers should return false if the target AsmParser wishes to
handle the directive, true otherwise.  Many of the target specific directive
parsers would 'return Error' which does not follow these semantics.  The result
was noisy diagnostics which would also be slightly misleading.  This change
simply changes the target specific routine to conform to the semantics of
ParseDirective properly.
ARM IAS: support .personalityindex

The .personalityindex directive is equivalent to the .personality directive with
the ARM EABI personality with the specific index (0, 1, 2).  Both of these
directives indicate personality routines, so enhance the personality directive
handling to take into account personalityindex.

Bonus fix: flush the UnwindContext at the beginning of a new function.
ARM IAS: support .movsp

.movsp is an ARM unwinding directive that indicates to the unwinder that a
register contains an offset from the current stack pointer.  If the offset is
unspecified, it defaults to zero.