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.

1 files changed, 5 insertions, 0 deletions

diff --git a/lib/std/start.zig b/lib/std/start.zig
index d8e5796d37..057fde62f6 100644
--- a/lib/std/start.zig
+++ b/lib/std/start.zig
@@ -87,6 +87,11 @@ fn main2() callconv(.C) c_int {
 }
 
 fn _start2() callconv(.Naked) noreturn {
+    callMain2();
+}
+
+fn callMain2() noreturn {
+    @setAlignStack(16);
     root.main();
     exit2(0);
 }