stage2: prepare for building freestanding libc

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

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