Change signature of __builtin_rotateright64 back to unsigned

The signature of __builtin_rotateright64 was by misstake changed from
unsigned to signed in r360863, this patch will change it back to
unsigned as intended.

This fixes pr43309

Reviewers: efriedma, hans

Reviewed By: hans

Differential Revision: https://reviews.llvm.org/D67606

llvm-svn: 371969