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

[Alex Bradbury via llvm-commits]

asb wrote:

In terms of benchmarking, I'd set off a run on the SpacemiT K1 but before I'd matched AArch64 in disabling merging for external globals. For the specrate benchmarks that I'd been running, the mean 500.perlbench_r runtime is ~0.7% higher after this change. gcc_r is perhaps a coulple of tenths of a percent faster after the change, but in both cases I'd need to do a proper statistical test beyond just looking at the mean and standard deviation.

I'll kick off a run for this precise change, but my expectation is very minimal impact indeed (if people have particular benchmarks in mind that may be impacted, that would be very welcome). As noted in the above commit, I think minimal impact would be fine - we'd be more aligned to AArch64, and can look to iteratively improve the impact of globals merging.

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