[SPARC] Prefer RDPC over CALL to implement GETPCX for 64-bit target

On 64-bit target, prefer using RDPC over CALL to get the value of %pc.
This is faster on modern processors (Niagara T1 and newer) and avoids
polluting the processor's predictor state.

The old behavior of using a fake CALL is still done when tuning for
classic UltraSPARC processors, since RDPC is much slower there.

A quick pgbench test on a SPARC T4 shows about 2% speedup on SELECT
loads, and about 7% speedup on INSERT/UPDATE loads.

Reviewed By: @s-barannikov

Github PR: https://github.com/llvm/llvm-project/pull/78280