Using __builtin_expect (likely/unlikely macros)

[Alex Gramiak]

Would using these, if available, be acceptable? I noticed that some code
in configure.ac already defines HAVE___BUILTIN_EXPECT, though I couldn't
see what part of autoconf does this.

Good candidates here include the conditionals around emacs_abort/fatal,
and possibly in cases like the *alloc procedures and xfree.

Even if there's no noticeable speedup, I think that the likely/unlikely
macros are a nice way to indicate that a branch is exceedingly
rare/common.

  #define unlikely(expr) __builtin_expect(!!(expr), 0)
  #define likely(expr) __builtin_expect(!!(expr), 1)