[llvm] [Exegesis] CPU selection, when native arch and target mismatch (PR #131014)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-nvptx-nvidia-ubuntu` running on `as-builder-7` while building `llvm` at step 6 "test-build-unified-tree-check-llvm".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/180/builds/15396

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 6 (test-build-unified-tree-check-llvm) failure: test (failure)
******************** TEST 'LLVM :: tools/llvm-exegesis/X86/mcpu_not_set_during_cross_compilation.s' FAILED ********************
Exit Code: 1

Command Output (stderr):
--
not /home/buildbot/worker/as-builder-7/ramdisk/llvm-nvptx-nvidia-ubuntu/build/bin/llvm-exegesis -mtriple=riscv64-unknown-linux-gnu -mode=latency --benchmark-phase=assemble-measured-code -opcode-name=ADD 2>&1 | /home/buildbot/worker/as-builder-7/ramdisk/llvm-nvptx-nvidia-ubuntu/build/bin/FileCheck /home/buildbot/worker/as-builder-7/ramdisk/llvm-nvptx-nvidia-ubuntu/llvm-project/llvm/test/tools/llvm-exegesis/X86/mcpu_not_set_during_cross_compilation.s # RUN: at line 1
+ not /home/buildbot/worker/as-builder-7/ramdisk/llvm-nvptx-nvidia-ubuntu/build/bin/llvm-exegesis -mtriple=riscv64-unknown-linux-gnu -mode=latency --benchmark-phase=assemble-measured-code -opcode-name=ADD
+ /home/buildbot/worker/as-builder-7/ramdisk/llvm-nvptx-nvidia-ubuntu/build/bin/FileCheck /home/buildbot/worker/as-builder-7/ramdisk/llvm-nvptx-nvidia-ubuntu/llvm-project/llvm/test/tools/llvm-exegesis/X86/mcpu_not_set_during_cross_compilation.s
/home/buildbot/worker/as-builder-7/ramdisk/llvm-nvptx-nvidia-ubuntu/llvm-project/llvm/test/tools/llvm-exegesis/X86/mcpu_not_set_during_cross_compilation.s:3:10: error: CHECK: expected string not found in input
# CHECK: llvm-exegesis error: A CPU must be explicitly specified when cross compiling. To see all possible options for riscv64-unknown-linux-gnu triple use -mcpu=help
         ^
<stdin>:1:1: note: scanning from here
llvm-exegesis error: no LLVM target for triple riscv64-unknown-linux-gnu
^

Input file: <stdin>
Check file: /home/buildbot/worker/as-builder-7/ramdisk/llvm-nvptx-nvidia-ubuntu/llvm-project/llvm/test/tools/llvm-exegesis/X86/mcpu_not_set_during_cross_compilation.s

-dump-input=help explains the following input dump.

Input was:
<<<<<<
         1: llvm-exegesis error: no LLVM target for triple riscv64-unknown-linux-gnu 
check:3     X~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ error: no match found
>>>>>>

--

********************


```

</details>

https://github.com/llvm/llvm-project/pull/131014