Prototype compiler builtins.
Prototype compiler builtins.

Add compatible prototypes for compiler builtins
__builtin_unreachable, __builtin_ctzl, __builtin_clzl,
and __builtin_popcountl.

The compiler ignores these, but they are necessary for the Isabelle
C parser to handle them. This is needed to drop DONT_TRANSLATE markers
from various functions which call these builtins.

cmr/seL4,cmr/seL4,cmr/seL4