[Sparc] Add support for the cycle counter available in GR740

[Sparc] Add support for the cycle counter available in GR740

Summary: The GR740 provides an up cycle counter in the registers ASR22
and ASR23. As these registers can not be read together atomically we only
use the value of ASR23 for llvm.readcyclecounter(). The ASR23 register
holds the 32 LSBs of the up-counter.

Reviewers: jyknight, venkatra

Reviewed By: jyknight

Subscribers: jfb, fedor.sergeev, jrtc27, llvm-commits

Differential Revision: https://reviews.llvm.org/D48638

llvm-svn: 340733