Some old assemblers can't cope with r0 in address. It's actually
sensible thing to do, because r0 is shunted to 0 in address arithmetic
and by refusing r0 assembler effectively makes you understand that.

(Imported from upstream's a54aba531327285f64cf13a909bc129e9f9d5970.)

This also pulls in a trailing whitespace fix from upstream's
609b0852e4.

Change-Id: Ieec0bc8d24b98f86ce4fc9ee6ce5126d127cf452
Reviewed-on: https://boringssl-review.googlesource.com/12188
Reviewed-by: Adam Langley <agl@google.com>