[llvm] [BOLT] Use llvm-nm by default on Windows (PR #151805)

[Paschalis Mpeis via llvm-commits]

https://github.com/paschalis-mpeis approved this pull request.

Looks good, thanks. I'm curious what @aaupov and @yota9 think, as we plan to make it the default for all platforms, correct?

IIRC, we had a test that relied on GNU `nm --synthetic` because `llvm-nm --special-syms` didn't do it? However, [that part of the test is disabled](https://github.com/llvm/llvm-project/blob/7085065c02da6091dca91be201160912e43a63ec/bolt/test/X86/callcont-fallthru.s#L17) until #138232 by @yota9 is merged.

Is there anything else holding us back from making `llvm-nm` the default on all platforms? Linux AArch64 appears to be ready for this too.

https://github.com/llvm/llvm-project/pull/151805