stage2: implement @setAlignStack and 128-bit cmpxchg

* test runner is improved to respect `error.SkipZigTest`
 * start code is improved to `@setAlignStack(16)` before calling main()
 * the newly passing behavior test has a workaround for the fact that
   stage2 cannot yet call `std.Target.x86.featureSetHas()` at comptime.
   This is blocking on comptime closures. The workaround is that there
   is a new decl `@import("builtin").stage2_x86_cx16` which is a `bool`.
 * Implement `@setAlignStack`. This language feature should be re-evaluated
   at some point - I'll file an issue for it.
 * LLVM backend: apply/remove the cold attribute and noinline attribute
   where appropriate.
 * LLVM backend: loads and stores are properly annotated with alignment
   and volatile attributes.
 * LLVM backend: allocas are properly annotated with alignment.
 * Type: fix integers reporting wrong alignment for 256-bit integers and
   beyond. Once you get to 16 byte aligned, there is no further
   alignment for larger integers.

src/codegen/llvm/bindings.zig

@@ -85,6 +85,9 @@ pub const Value = opaque {
     pub const addAttributeAtIndex = LLVMAddAttributeAtIndex;
     extern fn LLVMAddAttributeAtIndex(*const Value, Idx: AttributeIndex, A: *const Attribute) void;
 
+    pub const removeEnumAttributeAtIndex = LLVMRemoveEnumAttributeAtIndex;
+    extern fn LLVMRemoveEnumAttributeAtIndex(F: *const Value, Idx: AttributeIndex, KindID: c_uint) void;
+
     pub const getFirstBasicBlock = LLVMGetFirstBasicBlock;
     extern fn LLVMGetFirstBasicBlock(Fn: *const Value) ?*const BasicBlock;
 
@@ -136,6 +139,12 @@ pub const Value = opaque {
 
     pub const setOrdering = LLVMSetOrdering;
     extern fn LLVMSetOrdering(MemoryAccessInst: *const Value, Ordering: AtomicOrdering) void;
+
+    pub const setVolatile = LLVMSetVolatile;
+    extern fn LLVMSetVolatile(MemoryAccessInst: *const Value, IsVolatile: Bool) void;
+
+    pub const setAlignment = LLVMSetAlignment;
+    extern fn LLVMSetAlignment(V: *const Value, Bytes: c_uint) void;
 };
 
 pub const Type = opaque {