Add volatile qualifications to two blocks of inline asm to stop GCC from
eliminating them as dead code.

Both volatile and "memory" are used because of some concern that the compiler
may still cache values across the asm block without it, and because this was
such a painful debugging session that I wanted to ensure that it's never
repeated.

Patch by Adam Langley.
Remove return value from HOST_(c2l|l2c) macros.

The reason for this change is because every where they called
these macros they casted the return value to void. This will
also give a speed boost.

Largely based on a change by libressl.