Provide vtohl, vtohs, htovl and htovs no-op macros on little endian systems.

Provide vtohl, vtohs, htovl and htovs no-op macros on little endian systems.

This means that there are always macros or functions for vtohl, vtohs,
htovl and htovs available, so eliminate HAS_VTOHL etc, and unconditionally
compile the code that it was protecting. grep.cpan.me shows that no code on
CPAN uses any of these macros.

(Technically the 4 are not quite no-ops, as they truncate their values to
32 or 16 bits, to be consistent with the implementations for platforms which
need re-ordering.)