[lld] [RISCV][LLD] Add RISCV zcmt optimise in linker relaxation (PR #77884)

[Alex Bradbury via llvm-commits]

asb wrote:

The RISC-V code size reduction work-group did some analysis of expected code size improvements from zcmt <https://docs.google.com/spreadsheets/d/1bFMyGkuuulBXuIaMsjBINoCWoLwObr1l9h5TAWN8s7k/edit#gid=1679419155> - it would be worth understanding why the results seem so different. 

https://github.com/llvm/llvm-project/pull/77884