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

[Fangrui Song via llvm-commits]

MaskRay wrote:

@aaupov 

IIUC the issue was that premerge bots and https://lab.llvm.org/buildbot/#/builders/272 disagreed on what `clang` to use.
Before 59bfc31068746dcae4cfb3cc648cd5cf243a3ccd `[CI] Use trunk Clang in BOLT testing`, I saw `/usr/bin/clang` used in one and `path/to/build/bin/clang` in another.

This discrepancy would cause `bolt/test/RISCV/fake-label-no-entry.c` to fail in one of the bots.

We should remove these `/usr/bin/clang` settings as otherwise the bolt tests would be too sensitive to LLVMMC changes.

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