Remove invalid const qualifier for r0, since it is written by svc.

Summary: This causes an invalid lvalue error when building for Android.

Reviewers: danalbert

Reviewed By: danalbert

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D7306

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@227907 91177308-0d34-0410-b5e6-96231b3b80d8