added two builtin options:

--enable-builtin-kinds k1,k2
--disable-builtin-kinds k1,k2

enable or disable certain builtins.
Currently, all builtins such as:
  "UInt; __builtin_bswap32; (UInt)"
will be enabled by default.

Builtins with a ``kind'' flag will be enabled only if
``--enable-builtin-kinds kind'' is passed, e.g.

"Int; __builtin_ctzs; (UShort); clang",
"UShort; __builtin_bswap16; (UShort); ppc"

passing ``--enable-builtin-kinds clang,pcc'' will enable
both __builtin_ctzs and __builtin_bswap16, whereas
passing ``--enable-builtin-kinds clang'' will enable
only __builtin_ctzs.

All default builtins, i.e. those without ``kind'' flag,
can be disabled by pass ``--disable-builtin-kinds generic''
Merge branch 'master' of github.com:csmith-project/csmith
added a bunch of builtins

Thanks Hal Finkel for providing the list!
added help msg for enable and disable builtin-kinds
raise the default probability of choosing builtins to 50