Extracts lib/std/special/c_stage1.zig from lib/std/special/c.zig.
When the self-hosted compiler is further along, all the logic from c_stage1.zig will
be migrated back c.zig and then c_stage1.zig will be deleted. Until then we have a
simpler implementation of c.zig that only uses features already implemented in self-hosted.
So far it only contains memcpy and memset, with slightly different
(arguably more correct!) implementations that are compatible with
self-hosted.
Additionally, this commit improves the LLVM backend:
* use the more efficient and convenient fnInfo() when lowering function
type info.
* fix incremental compilation not deleting all basic blocks of a
function.
* hook up calling conventions
* hook up the following function attributes:
- noredzone, nounwind, uwtable, minsize, optsize, sanitize_thread
418105589a