remove return value from HOST_c2l/l2c macros

These macros and asm inlines simulate a function returning a value, but
nothing ever uses this return value. Remove the pseudo-returns and
(void) casts discarding the unused values.

This, maybe unsurprisingly, speeds things up a bit. It also removes the
GCC 4.9 warnings about unused values.

ok miod@ deraadt@
replace more ROTATE macros with plain-old C code.

Let the compiler optimize these. Even older versions of gcc generate
equal or better quality code than the inline asm.

ok miod@