compiler-rt: remove dead code

`__addosi4`, `__addodi4`, `__addoti4`, `__subosi4`, `__subodi4`, and
`__suboti4` were all functions which we invented for no apparent reason.
Neither LLVM, nor GCC, nor the Zig compiler use these functions. It
appears the functions were created in a kind of misunderstanding of an
old language proposal; see https://github.com/ziglang/zig/pull/10824.

There is no benefit to these functions existing; if a Zig compiler
backend needs this operation, it is trivial to implement, and *far*
simpler than calling a compiler-rt routine. Therefore, this commit
deletes them. A small amount of that code was used by other parts of
compiler-rt; the logic is trivial so has just been inlined where needed.
I also chose to quickly implement `__addvdi3` (a standard function)
because it is trivial and we already implement the `sub` parallel.

CMakeLists.txt

@@ -211,10 +211,10 @@ set(ZIG_STAGE2_SOURCES
     lib/compiler_rt/absvti2.zig
     lib/compiler_rt/adddf3.zig
     lib/compiler_rt/addf3.zig
-    lib/compiler_rt/addo.zig
     lib/compiler_rt/addsf3.zig
     lib/compiler_rt/addtf3.zig
     lib/compiler_rt/addvsi3.zig
+    lib/compiler_rt/addvdi3.zig
     lib/compiler_rt/addxf3.zig
     lib/compiler_rt/arm.zig
     lib/compiler_rt/atomics.zig
@@ -354,7 +354,6 @@ set(ZIG_STAGE2_SOURCES
     lib/compiler_rt/sqrt.zig
     lib/compiler_rt/stack_probe.zig
     lib/compiler_rt/subdf3.zig
-    lib/compiler_rt/subo.zig
     lib/compiler_rt/subsf3.zig
     lib/compiler_rt/subtf3.zig
     lib/compiler_rt/subvdi3.zig