Prevent CPU HLO profiling crashes

[stevenvar]

This PR fixes a crash when using --xla_hlo_profile=true with XLA:CPU.

Before this change, XLA could generate profiling code, but at runtime the profiling counter buffer could be missing. The generated code then tried to write to a null pointer and crashed.

This change:

• creates the profiling counter buffer before running the compiled CPU function
• passes that buffer to the generated code
• adds a safety check so profiling does not crash if the buffer is missing
• disables HLO profiling for the CPU thunk runtime, because that path does not support profiling counters yet

On non-x86 CPUs, it also uses a safer runtime timer instead of a low-level LLVM cycle counter.