[llvm] [BOLT] Fix NOP instruction emission on x86 (PR #72186)

[Maksim Panchenko via llvm-commits]

maksfb wrote:

Note that with this change, NOP instruction encoding could be different. E.g., 4-byte NOP is now encoded as:

    0f 1f 40 00
compared to:

    0f 1f 04 00
previously.

This is also required to match Linux kernel expectations for NOP encoding.

https://github.com/llvm/llvm-project/pull/72186