Change the arm assembler to support this from the v7c spec:
"When assembling to the ARM instruction set, the .N qualifier produces
an assembler error and the .W qualifier has no effect."

In the pre-matcher handler in the asm parser the ".w" (wide) qualifier
when in ARM mode is now discarded. And an error message is now
produced when the ".n" (narrow) qualifier is used in ARM mode.

Test cases for these were added.

rdar://14064574