use the "symbol" helper function in all exports
move all declarations from common.zig to compiler_rt.zig
flatten the tree structure somewhat (move contents of tiny files into
parent files)
No functional changes.
Most of this migration was performed automatically with `zig fmt`. There
were a few exceptions which I had to manually fix:
* `@alignCast` and `@addrSpaceCast` cannot be automatically rewritten
* `@truncate`'s fixup is incorrect for vectors
* Test cases are not formatted, and their error locations change
