[lld] [NFC][lld][ELF] move "initSymbolAnchors" to "lld/ELF/Writer.cpp" (PR #101869)

[Fangrui Song via llvm-commits]

MaskRay wrote:

This does not seem necessary. `initSymbolAnchors` in RISCV.cpp makes linker relaxation code together.
Moving the function to `Writer.cpp` would make the code harder to read.

https://github.com/llvm/llvm-project/pull/101869