Add volatile qualifications to two blocks of inline asm to stop GCC from

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.
(cherry picked from commit 7753a3a68431aa81b82beea4c3f5374b41454679)