Multiple TCs

My take on adding multiple TCs. Feedback welcome @SynoCommunity/developers.

Not done yet: update SPK's using $(ARCH) comparison, add kernels for DSM5, possibly add some way of using/setting a default toolchain version.
Spkrepo might need an update, as the SPK name includes the TC version too.

Ref #983 
Yeah, I was thinking to get rid of the call from TCVERSION, and just put the code directly in the rules in `cross-cc.mk` and `spk.mk`.