GCC: remove unneeded asm volatiles

GCC: remove unneeded asm volatiles

Many assembler intrinsics are pure - they read only the input registers,
write only the output registers, and change no PSR flags.

Remove volatile from all such intrinisics to improve potential
optimisation.

   REV, REV16, REVSH, RBIT
   QADD8, SHADD8, UQADD8, UHADD8
   QSUB8, SHSUB8, UQSUB8, UHSUB8
   QADD16, SHADD16, UQADD16, UHADD16
   QSUB16, SHSUB16, UQSUB16, UHSUB16
   QASX, SHASX, UQASX, UHASX
   QSAX, SHSAX, UQSAX, UHSAX
   USAD8, USADA8
   UXTB16, UXTAB16, SXTB16, SXTAB16
   SMMLA

Change-Id: Id7e245e621dfc78b027db89f073102cf99d92174