[ARM, gas] Don't warn on REG_SP when used in CRC32 instructions

[Nick Clifton]

Hi Jiong,

>    OK for master and backport to 2.28 and 2.29?

Approved for master and the 2.29 branch.

You can backport it to the 2.28 branch if you really want to, 
but we are not going to be making any more releases from there...

Cheers
  Nick