Current GCC versions know how to generate these instructions
properly and avoiding inline asm gives better code.  The MULH
function for ARMv5 uses the same instruction and is also not
needed any more.

The MLS64 macro remains since negating an input would normally
not be allowed as it would fail for INT_MIN.  In our uses, the
inputs never have this value and thus negating is safe.

Signed-off-by: Mans Rullgard <mans@mansr.com>