gallium/auxiliary: add inc and dec alternative with return - continue

At this moment we use only >= 0 values.

() Implement it for GCC X86(_64), MSVC ASM and Solaris.
   MSVC and Solaris is untested.

Question is: Solaris use uint (which is fine for us) but others use int.
What to do?

Signed-off-by: David Heidelberger <david.heidelberger@ixit.cz>