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

[Alex Bradbury via llvm-commits]

asb wrote:

I've had a look at this + Dhrystone and this change actually makes zero codegen difference for it at all. This is the same as for AArch64, and due to MergeExternalByDefault being False. It does kick in if that is force enabled (and in the default configuration for Arm), so this is obvious a benchmark to look at more closely for the next step after this lands - looking to enable that option.

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