Move the ARM opcode macros from <target/armv4_5.h>, and a few
Thumb2 ones from <target/armv7m.h>, to more appropriate homes
in a new <target/arm_opcodes.h> file.

Removed duplicate opcodes from that v7m/Thumb2 set.  Protected
a few macro argument references by adding missing parentheses.

Tightening up some of the line lengths turned up a curious artifact:
the macros for the Thumb opcodes are all 32 bits wide, not 16 bits.
There's currently no explanation for why it's done that way...

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>