[lld] [llvm] [MC] Rename temporary symbols of empty name to ".L0 " (PR #89693)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> @MaskRay – can this be relanded now?

I relanded this patch as it should actually fix `bolt/test/RISCV/fake-label-no-entry.c`.

`bolt/test/RISCV/fake-label-no-entry.c` is too sensitive to clang versions. It can probably be removed if we still see failures.
The original purpose that `bolt/test/RISCV/fake-label-no-entry.c` was contributed/worked around was properly fixed by this patch anyway.

https://github.com/llvm/llvm-project/pull/89693