[llvm] [NFC][llvm-exegesis] Disable CFI-icall for JIT-executed function (PR #202472)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

This produces quite noisy warnings when building with GCC. (Yes, building with GCC isn't anywhere near warning clean anyway, but I'd try to avoid piling on more if avoidable.)

The warnings I see are copies of this, for each time the header is included:
```
In file included from ../tools/llvm-exegesis/lib/BenchmarkRunner.h:18,
                 from ../tools/llvm-exegesis/lib/BenchmarkRunner.cpp:9:
../tools/llvm-exegesis/lib/Assembler.h:118:47: warning: ‘cfi-icall’ attribute directive ignored [-Wattributes]
  118 |       __attribute__((no_sanitize("cfi-icall"))) /* Incompatible with JIT
 */ {
      |                                               ^
```

https://github.com/llvm/llvm-project/pull/202472