From: Arnd Bergmann <arnd@arndb.de>
Date: Wed, 25 May 2016 22:47:33 +0200

If we use the normal calling conventions, we could remove these overrides
along with the respective special-case handling in glibc. None of them
look particularly performance-sensitive, but I could be wrong there.

You could set the lowest bit in the system call entry pointer to indicate
the upper-half clears should be elided.