undefine internal-use type macros at the end of alltypes.h

undefine internal-use type macros at the end of alltypes.h

this patch is something of a compromise for a compatibility
regression discovered after the header refactoring: libtiff uses
_Int64 for its own use. this is absolutely wrong, invalid C, and
should not be supported, but it's also frustrating for users when code
that used to work suddenly breaks.

rather than leave the breakage in place or change musl internals to
accommodate broken software, I've found a change that makes the
problem go away and improves musl. by undefining these macros at the
end of alltypes.h, the temptation to use them in other headers is
removed. (for example, I almost used _Int64 in sys/types.h to define
u_int64_t rather than adding it back to alltypes.h.) by confining use
of these macros to alltypes.h, we keep it easy to go back and change
the implementation of alltypes later, if needed.