* move SPU code from std to self hosted compiler
* change std lib comments to be descriptive rather than prescriptive
* avoid usingnamespace
* fix case style of error codes
* remove duplication of producer_string
* generalize handling of less than 64 bit arch pointers
* clean up SPU II related test harness code
See #6113 for an alternate way of doing this that we didn't end up
following.
Closes#6079.
I also took the opportunity here to extract C.zig and Elf.zig from
link.zig.
