[llvm] [RISCV] Enable global merging by default (PR #115495)

[Philip Reames via llvm-commits]

https://github.com/preames approved this pull request.

LGTM because the discussion from round table was clearly in this direction and the change is straight forward.  However, please wait a couple of days for other input and in particular, for any suggestions on blocking benchmarking.  (I would recommend measuring drystone since it tends to be very sensitive to gp-relaxation.)

https://github.com/llvm/llvm-project/pull/115495