base/asm-common.h: Add an `IMM' macro for immediate operands.

The most useful version of this is `IMM(r, ...)', because that varies
according to the target architecture, but the others might be considered
an improvement over the Intel syntax.

It turns out that I don't actually need this: the motivation was caused
by a typo-ed register field.  But I think it still makes a useful
addition.