Add hi16[], ha16[], lo16[] support to the PowerPC assembler

[davidgiven]

This adds both assembler support and backend linker support for Apple-style hi16[], ha16[], lo16[] expressions which can be used in instructions which take 16-bit constant operands.

This allows more compact instructions when doing things like loads from constant addresses, using syntax like this:

addis r3, r0, ha16[value+88]
lwz r3, r3, lo16[value+88]

Also clean up led a bit for more standard use of system #includes.

[kernigh]

This part from mach/powerpc/ncg/table looks wrong:

	pat loi $1==INT32                  /* Load word indirect */
		with LABEL
			uses REG
			gen
				lis %a, {LABEL_HA, %1.adr}
				lwz %a, {GPRINDIRECT_OFFSET_LO, %a, %1.adr}
			yields %a

I believe that we can't yield a register here, because it isn't a token in the MEMORY set. I'm currently thinking about how to rewrite this.

[kernigh]

My last comment was wrong. We can yield a register because the lwz did read the memory. We don't need to kill the register if we change the value in memory. This is different from a token like IND_RC_W. The token would delay reading the memory, and we would need to kill the token before we changed the value in memory, because we must read the value before we change it.

In fact, we can't yield a token here, because our assembler needs lo16 in the next instruction after ha16. So we can't yield a token that would delay the lo16.